Search

Many established models of animal foraging assume that individuals are ecologically equivalent. However, it is increasingly recognized that populations may comprise individuals who differ consistently in their diets and foraging behaviors. For example, recent studies have shown that individual foraging site fidelity (IFSF, when individuals consistently forage in only a small part of their population's home range) occurs in some colonial breeders. Short-term IFSF could result from animals using a win-stay, lose-shift foraging strategy. Alternatively, it may be a consequence of individual specialization. Pelagic seabirds are colonial central-place foragers, classically assumed to use flexible foraging strategies to target widely dispersed, spatiotemporally patchy prey. However, tracking has shown that IFSF occurs in many seabirds, although it is not known whether this persists across years. To test for long-term IFSF and to examine alternative hypotheses concerning its cause, we repeatedly tracked 55 Northern Gannets (Morus bassanus) from a large colony in the North Sea within and across three successive breeding seasons. Gannets foraged in neritic waters, predictably structured by tidal mixing and thermal stratification, but subject to stochastic, wind-induced overturning. Both within and across years, coarse to mesoscale (tens of kilometers) IFSF was significant but not absolute, and foraging birds departed the colony in individually consistent directions. Carbon stable isotope ratios in gannet blood tissues were repeatable within years and nitrogen ratios were also repeatable across years, suggesting long-term individual dietary specialization. Individuals were also consistent across years in habitat use with respect to relative sea surface temperature and in some dive metrics, yet none of these factors accounted for IFSF. Moreover, at the scale of weeks, IFSF did not decay over time and the magnitude of IFSF across years was similar to that within years, suggesting that IFSF is not primarily the result of win-stay, lose-shift foraging. Rather, we hypothesize that site familiarity, accrued early in-life, causes IFSF by canalizing subsequent foraging decisions. Evidence from this and other studies suggests that IFSF may be common in colonial central-place foragers, with far-reaching consequences for our attempts to understand and conserve these animals in a rapidly changing environment.

Foraging decisions by native grazers in fire-dependent landscapes modulate the fire–grazing interaction. Uncovering the behavioral mechanisms associated with the attraction of grazers to recently burned areas requires understanding at multiple spatial scales in the ecological foraging hierarchy. This study focused on feeding in the area between steps in a foraging bout, the feeding station, as forage chemistry and vegetation architecture play central roles in these fine-scale, feeding-station decisions. The forage maturation hypothesis (FMH) uses the temporal dynamics of forage quality and quantity in grasslands to explain the distribution of large herbivores, but does not address herbivore responses to inter-patch variation caused by fire-induced nutrient increases of forage quality. Using an experimental setting with contrasting fire treatments we describe the effects of variable burn history on foraging kinetics by bison at Konza Prairie Biological Station (KPBS). We assessed the potential to link the ...

In mixed-species flocks, the heavier tit species forage in the inner tree parts, while the smaller species exploit food items of the outer canopy. Using aviary experiments we studied the relative importance of two possible explanations for body size-foraging site correlation. The first hypothesis is that larger bodied tits are more efficient than smaller ones in exploiting the inner tree parts, while the reverse would be true in the outer canopy. However, the heavier species, the Willow Tit Parus montanus, was not more efficient in food searching in the inner tree parts than the smaller species, the Coal Tit Parus ater. The second hypothesis is based on interference competition with the socially dominant (heavier) species selecting the most profitable foraging sites, and forcing the smaller species to less rewarding places. In fact, in confrontation experiments only individuals of the smaller species changed their foraging sites in the presence of an individual of the other species in the aviary. In sum, body size influences social dominance relations and thereby relegates the smaller species to poorer foraging sites, but presumably gives them an advantage in exploitation competition via reduced energy demands for basal metabolism.

TM and TP received support from IMBER-CLIOTOP and Macquarie University Safety Net Grant 9201401743. SB was supported under an Australia Research Council Super Science Fellowship. IDJ was supported by a Macquarie Vice-Chancellors Innovation Fellowship. TAP was supported by a CSIRO Julius Career Award and the Villum Foundation. The behavior of colony-based marine predators is the focus of much research globally. Large telemetry and tracking data sets have been collected for this group of animals, and are accompanied by many empirical studies that seek to segment tracks in some useful way, as well as theoretical studies of optimal foraging strategies. However, relatively few studies have detailed statistical methods for inferring behaviors in central place foraging trips. In this paper we describe an approach based on hidden Markov models, which splits foraging trips into segments labeled as "outbound", "search", "forage", and "inbound". By structuring the hidden Markov model transition matrix appropriately, the model naturally handles the sequence of behaviors within a foraging trip. Additionally, by structuring the model in this way, we are able to develop realistic simulations from the fitted model. We demonstrate our approach on data from southern elephant seals (Mirounga leonina) tagged on Kerguelen Island in the Southern Ocean. We discuss the differences between our 4-state model and the widely used 2-state model, and the advantages and disadvantages of employing a more complex model. Publisher PDF

The costs and benefits of alternative migratory strategies are often framed in the context of top-down and bottom-up effects on individual fitness. This occurs because migration is considered a costly behavioral strategy that presumably confers explicit benefits to migrants in the form of either decreased predation risk (predation risk avoidance hypothesis) or increased nutrition (forage maturation hypothesis). To test these hypotheses, we studied a partially migratory moose (Alces alces) population and contrasted explicit measures of predation risk (i.e., offspring survival) and nutrition (i.e., accumulation of endogenous energy reserves) between resident and migratory subpopulations. We relied on data collected from migratory and nonmigratory radio-marked moose (n = 67) that inhabited a novel study system located in coastal Alaska between 2004 and 2010. In this area, 30% of the population resides year-round on a coastal foreland area, while 48% migrate to either a small island archipelago or a subalpine ridge system (the remainder exhibited one of six different low-occurrence strategies). Overall, we determined that accumulation of body fat during the growing season did not differ between migratory or resident modalities. However, calf survival was 2.6-2.9 times higher for individuals that migrated (survival, islands = 0.49 +/- 0.16 [mean +/- SE], n = 35; ridge = 0.52 +/- 0.16, n = 33) than those that did not (survival, resident = 0.19 +/- 0.08, n = 57). Our results support the predation risk avoidance hypotheses, and suggest that migration is a behavioral strategy that principally operates to reduce the risk of calf predation and does not confer explicit nutritional benefits. We did not directly detect trade-offs between predation risk and nutrition for migratory individuals. Yet we identify an indirect life history mechanism that may mildly dampen the apparent fitness benefits of migration. The proximate factors accounting for differences in migration-specific neonate survival are likely linked to accessibility of refugial habitats for moose at local and landscape scales, landscape factors that affect hunting efficacy of large carnivores, and interactions with rural human communities. Conservation of ungulate populations can be aided by integrating knowledge about migratory behavior, life history strategies, and factors that alter ungulate vulnerability, particularly those induced by human activity.

Plant species can influence the pollination and reproductive success of coflowering neighbors that share pollinators. Because some individual pollinators habitually forage in particular areas, it is also possible that plant species could influence the pollination of neighbors that bloom later. When flowers of a preferred forage plant decline in an area, site-fidelity may cause individual flower feeders to stay in an area and switch plant species rather than search for preferred plants in a new location. A newly blooming plant species may quickly inherit a set of visitors from a prior plant species, and therefore experience higher pollination success than it would in an area where the first species never bloomed. To test this, we manipulated the placement and timing of two plant species, Delphinium barbeyi and later-blooming Gentiana parryi. We recorded the responses of individually marked bumble bee pollinators. About 63% of marked individuals returned repeatedly to the same areas to forage on Delphinium. When Delphinium was experimentally taken out of bloom, most of those site-faithful individuals (78%) stayed and switched to Gentiana. Consequently, Gentiana flowers received more visits in areas where Delphinium had previously flowered, compared to areas where Delphinium was still flowering or never occurred. Gentiana stigmas received more pollen in areas where Delphinium disappeared than where it never bloomed, indicating that Delphinium increases the pollination of Gentiana when they are separated in time. Overall, we show that individual bumble bees are often site-faithful, causing one plant species to increase the pollination of another even when separated in time, which is a novel mechanism of pollination facilitation.

In animal populations, a minority of individuals consistently achieves the highest breeding success and therefore contributes the most recruits to future generations. On average, foraging performance is important in determining breeding success at the population level, but evidence is scarce to show that more successful breeders (better breeders) forage differently than less successful ones (poorer breeders). To test this hypothesis, we used a 10-year, three-colony, individual-based longitudinal data set on breeding success and foraging parameters of a long-lived bird, the Adélie Penguin, Pygoscelis adeliae. Better breeders foraged more efficiently than poorer breeders under harsh environmental conditions and when offspring needs were higher, therefore gaining higher net energy profit to be allocated to reproduction and survival. These results imply that adverse "extrinsic" conditions might select breeding individuals on the basis of their foraging ability. Adélie Penguins show sufficient phenotypic plasticity that at least a portion of the population is capable of surviving and successfully reproducing despite extreme variability in their physical and biological environment, variability that is likely to be associated with climate change and, ultimately, with the species' evolution. This study is the first to demonstrate the importance of "extrinsic" conditions (in terms of environmental conditions and offspring needs) on the relationship between foraging behavior and individual quality.

Trade-offs between predation risk and forage fundamentally drive resource selection by animals. Among migratory ungulates, trade-offs can occur at large spatial scales through migration, which allows an "escape" from predation, but trade-offs can also occur at finer spatial scales. Previous authors suggest that ungulates will avoid predation risk at the largest scale, although few studies have examined multi-scale trade-offs to test for the relative benefits of risk avoidance across scales. Building on previously developed spatial models of forage and wolf predation risk, we tested for trade-offs at the broad landscape scale and at a finer, within-home-range scale for migratory and non-migratory resident elk (Cervus elaphus) during summer in the Canadian Rockies in Banff National Park (BNP) and adjacent Alberta, Canada. Migration reduced exposure to wolf predation risk by 70% relative to residents at the landscape scale; at the fine scale, migrants used areas that were, on average, 6% higher in forage digestibility. In contrast, by forgoing migration, resident elk were exposed to higher predation risk, but they reduced predation risk at fine scales to only 15% higher than migrants by using areas close to human activity, which wolves avoided. Thus, residents paid for trying to avoid predation risk with lower forage quality. Residents may have been able to compensate, however, by using areas of abundant forage close to human activity where they may have been able to forage more selectively while avoiding predation risk. Human activity effectively decoupled the positive correlation between high forage quality and wolf predation, providing an effective alternate strategy for residents, similar to recent findings in other systems. Although ungulates appear capable of balancing risk and forage at different spatial scales, risk avoidance at large landscape scales may be more effective in the absence of human-caused refugia from predation.

For gregarious animals the cost-benefit trade-offs that drive habitat selection may vary dynamically with group size, which plays an important role in foraging and predator avoidance strategies. We examined how habitat selection by bison (Bison bison) varied as a function of group size and interpreted these patterns by testing whether habitat selection was more strongly driven by the competing demands of forage intake vs. predator avoidance behavior. We developed an analytical framework that integrated group size into resource selection functions (RSFs). These group-size-dependent RSFs were based on a matched case-control design and were estimated using conditional logistic regression (mixed and population-averaged models). Fitting RSF models to bison revealed that bison groups responded to multiple aspects of landscape heterogeneity and that selection varied seasonally and as a function of group size. For example, roads were selected in summer, but not in winter. Bison groups avoided areas of high snow water equivalent in winter. They selected areas composed of a large proportion of meadow area within a 700-m radius, and within those areas, bison selected meadows. Importantly, the strength of selection for meadows varied as a function of group size, with stronger selection being observed in larger groups. Hence the bison-habitat relationship depended in part on the dynamics of group formation and division. Group formation was most likely in meadows. In contrast, risk of group fission increased when bison moved into the forest and was higher during the time of day when movements are generally longer and more variable among individuals. We also found that stronger selection for meadows by large rather than small bison groups was caused by longer residence time in individual meadows by larger groups and that departure from meadows appears unlikely to result from a depression in food intake rate. These group-size-dependent patterns were consistent with the hypothesis that avoidance of predation risk is the strongest driver of habitat selection.

Animals selecting habitats often have to consider many factors, e.g., food and cover for safety. However, each habitat type often lacks an adequate mixture of these factors. Analyses of habitat selection using resource selection functions (RSFs) for animal radiotelemetry data typically ignore trade-offs, and the fact that these may change during an animal's daily foraging and resting rhythm on a short-term basis. This may lead to changes in the relative use of habitat types if availability differs among individual home ranges, called functional responses in habitat selection. Here, we identify such functional responses and their underlying behavioral mechanisms by estimating RSFs through mixed-effects logistic regression of telemetry data on 62 female red deer (Cervus elaphus) in Norway. Habitat selection changed with time of day and activity, suggesting a trade-off in habitat selection related to forage quantity or quality vs. shelter. Red deer frequently used pastures offering abundant forage and little canopy cover during nighttime when actively foraging, while spending much of their time in forested habitats with less forage but more cover during daytime when they are more often inactive. Selection for pastures was higher when availability was low and decreased with increasing availability. Moreover, we show for the first time that in the real world with forest habitats also containing some forage, there was both increasing selection of pastures (i.e., not proportional use) and reduced time spent in pastures (i.e., not constant time use) with lowered availability of pastures within the home range. Our study demonstrates that landscape-level habitat composition modifies the trade-off between food and cover for large herbivorous mammals. Consequently, landscapes are likely to differ in their vulnerability to crop damage and threat to biodiversity from grazing.

The forage maturation hypothesis (FMH) states that herbivores should follow the onset of growth in spring to obtain access to forage of higher quality and quantity, the so-called "green wave surfing." Several studies have found correlative evidence in support of this by associating animal movement with plant phenology. However, experimental manipulation of vast natural systems determining causes of large herbivore movement is usually beyond reach. The unique management system involving winter enclosures for wild red deer (Cervus elaphus) in Germany facilitated an opportunity for an experimental approach. We manipulated release dates of red deer into free-ranging conditions in spring, predicting increased overall access to high quality forage if released early (1 April), and more rapid initial movement speed towards higher elevation if released late (15 May). The latter had lower access to high quality forage than individuals released early, as they missed parts of the green wave. In strong support of the FMH, individuals released late moved at faster initial speed than early released individuals which tracked the green wave more closely, both settling when reaching similar elevations. This shows that red deer were flexible in their movements, and they can adapt to new patterns of phenology by phenotypic plasticity.

Attraction of mutualists and defense against antagonists are critical challenges for most organisms and can be especially acute for plants with pollinating and non-pollinating flower visitors. Secondary compounds in flowers have been hypothesized to adaptively mediate attraction of mutualists and defense against antagonists, but this hypothesis has rarely been tested. The tissues of milkweeds (Asclepias spp.) contain toxic cardenolides that have long been studied as chemical defenses against herbivores. Milkweed nectar also contains cardenolides, and we have examined the impact of manipulating cardenolides in nectar on the foraging choices of two flower visitors: generalist bumble bees, Bombus impatiens, which are mutualistic pollinators, and specialist monarch butterflies, Danaus plexippus, which are herbivores as larvae and ineffective pollinators as adults. Although individual bumble bees in single foraging bouts showed no avoidance of cardenolides at the highest natural concentrations reported for milkweeds, a pattern of deterrence did arise when entire colonies were allowed to forage for several days. Monarch butterflies were not deterred by the presence of cardenolides in nectar when foraging from flowers, but laid fewer eggs on plants paired with cardenolide-laced flowers compared to controls. Thus, although deterrence of bumble bees by cardenolides may only occur after extensive foraging, a primary effect of nectar cardenolides appears to be reduction of monarch butterfly oviposition.

Flexible time budgets allow individual animals to buffer the effects of variable food availability by allocating more time to foraging when food density decreases. This trait should be especially important for marine predators that forage on patchy and ephemeral food resources. We examined flexible time allocation by a long-lived marine predator, the Common Murre (Uria aalge), using data collected in a five-year study at three colonies in Alaska (USA) with contrasting environmental conditions. Annual hydroacoustic surveys revealed an order-of-magnitude variation in food density among the 15 colony-years of study. We used data on parental time budgets and local prey density to test predictions from two hypotheses: Hypothesis A, the colony attendance of seabirds varies nonlinearly with food density; and Hypothesis B, flexible time allocation of parent murres buffers chicks against variable food availability. Hypothesis A was supported; colony attendance by murres was positively correlated with food over a limited range of poor-to-moderate food densities, but independent of food over a broader range of higher densities. This is the first empirical evidence for a nonlinear response of a marine predator's time budget to changes in prey density. Predictions from Hypothesis B were largely supported: (1) chick-feeding rates were fairly constant over a wide range of densities and only dropped below 3.5 meals per day at the low end of prey density, and (2) there was a nonlinear relationship between chick-feeding rates and time spent at the colony, with chick-feeding rates only declining after time at the colony by the nonbrooding parent was reduced to a minimum. The ability of parents to adjust their foraging time by more than 2 h/d explains why they were able to maintain chick-feeding rates of more than 3.5 meals/d across a 10-fold range in local food density.

In animal ecology, a question of key interest for aquatic species is how changes in movement behavior are related in the horizontal and vertical dimensions when individuals forage. Alternative theoretical models and inconsistent empirical findings mean that this question remains unresolved. Here we tested expectations by incorporating the vertical dimension (dive information) when predicting switching between movement states ("resident" or "directed") within a state-space model. We integrated telemetry-based tracking and diving data available for four seal species (southern elephant, Weddell, antarctic fur, and crabeater) in East Antarctica. Where possible, we included dive variables derived from the relationships between (1) dive duration and depth (as a measure of effort), and (2) dive duration and the postdive surface interval (as a physiological measure of cost). Our results varied within and across species, but there was a general tendency for the probability of switching into "resident" state to be positively associated with shorter dive durations (for a given depth) and longer postdive surface intervals (for a given dive duration). Our results add to a growing body of literature suggesting that simplistic interpretations of optimal foraging theory based only on horizontal movements do not directly translate into the vertical dimension in dynamic marine environments. Analyses that incorporate at least two dimensions can test more sophisticated models of foraging behavior.

For many animals, selecting whether to forage during day or night is a critical fitness problem: at night, predation risks are lower but feeding is less efficient. Habitat selection is a closely related problem: the best location for nocturnal foraging could be too risky during daytime, and habitat that is safe and profitable in daytime may be unprofitable at night. We pose a theory that assumes animals select the combination of daytime and night activity (feeding vs. hiding), and habitat, that maximizes expected future fitness. Expected fitness is approximated as the predicted probability of surviving starvation and predation over a future time horizon, multiplied by a function representing the fitness benefits of growth. The theory's usefulness and generality were tested using pattern-oriented analysis of an individual-based model (IBM) of stream salmonids and the extensive literature on observed diel behavior patterns of these animals. Simulation experiments showed that the IBM reproduces eight diverse patterns observed in real populations. (1) Diel activity (whether foraging occurs during day and/or night) varies among a population's individuals, and from day to day for each individual. (2) Salmonids feed in shallower and slower water at night. (3) Individuals pack more tightly into the best habitat when feeding at night. (4) Salmonids feed relatively more at night if temperatures (and, therefore, metabolic demands) are low. (5) Daytime feeding is more common for life stages in which potential fitness increases more rapidly with growth. (6) Competition for feeding or hiding sites can shift foraging between day and night. (7) Daytime feeding is more common when food avail- ability is low. (8) Diel activity patterns are affected by the availability of good habitat for feeding or hiding. We can explain many patterns of variation in diel foraging behavior without assuming that populations or individuals vary in how inherently nocturnal or diurnal they are. Instead, these patterns can emerge from the search by individuals for good trade- offs between growth and survival under different habitat and competitive conditions.

We tested the fundamental assumption of the “optimality paradigm” that the foraging behavior of individual organisms corresponds to what we would expect if it had been honed by natural selection to match habitat differences in reproductive success. First, we used long-term studies of life history and habitat selection in white-footed mice to illustrate that the fitness of females living in the forest is greater than that of females living in forest-edge habitat. Second, we used short-term foraging studies to evaluate whether food patches located in the forest provided more value to foragers than did those in the edge. Third, we used foraging studies and data on the occurrence of predators to demonstrate that animals foraging in areas with little cover face higher risks than when they forage in areas with more cover. We confirmed three a priori predictions: (1) Individual mice abandoned foraging patches at higher harvest rates in edge habitat than they did in forest. (2) Individuals harvested resource patches to lower quitting harvest rates under cover than they did when patches were located in the open. (3) The difference in quitting-harvest rate between “open” and “covered” patches was less in the safe forest habitat than it was in the risky edge habitat. Our results yield an impressive fit with our previous knowledge of habitat differences in reproductive success and substantiate the premise that short-duration strategic decisions by individuals match habitat differences in fitness.

I studied the foraging behavior of Black-legged Kittiwakes (Rissa tridactyla) using radio transmitters, remote receiving stations, and a boat to track individuals. I tested the hypothesis that individual birds exhibit foraging area fidelity. I also investigated evidence for information transfer among birds and the role of tidal cycles in foraging behavior. Flight speed, feeding methods, chick growth rates, and fledging success were similar for radio-tagged birds and birds without radio-tags, suggesting that radio-tags had no significant effect. Birds made an average of two foraging trips per day for 13 d, during a spring and a neap tide series. Twenty-four of 26 birds returned repeatedly (X = 19.2 trips) to particular areas to feed; these birds were considered faithful to a foraging area. During spring and neap tides, 17 and 14 birds, respectively, timed their foraging trips to coincide with daily tidal cycles. Foraging area fidelity indicates that kittiwakes learn and remember when and where to forage an...

The simplest and arguably the most ubiquitous pattern in seed dispersal mutualisms is size coupling: large frugivores tend to consume larger fruits and small frugivores tend to consume smaller fruits. Despite the simplicity of this pattern, the potential mechanisms responsible for fruit--frugivore size coupling are mechanistically divergent and poorly resolved. Size coupling could arise deterministically, if large frugivores actively seek out larger fruits to maximize their foraging efficiency. Alternatively, size coupling could also arise passively, if frugivores forage randomly, but are able to consume only those fruit species that are smaller than their gape width. I observed birds forage for fruits in a New Zealand forest reserve at approximately five-day intervals for six years to test for fruit--frugivore size coupling. I then derived a suite of network analyses to establish whether fruit--frugivore size coupling was best explained by active or passive foraging by frugivores. Results showed a strikingly strong pattern in size coupling; the average size of fruits consumed by each frugivore species increased with their maximum gape width. Simulation analyses revealed that over 70% of variation in interaction frequencies in the observed fruit-frugivore web could be explained by a size-constrained, passive, foraging model. Foraging models in which birds foraged actively for different-sized fruits to improve their foraging efficiency performed more poorly. Results were therefore consistent with the hypothesis that apparently nonrandom patterns in seed dispersal mutualisms can sometimes arise from simple stochastic processes.

Why do birds partition habitats? Possible explanations include nonoverlap- ping food distributions, interspecific competition, and predation. If food limits distributions, species should forage most successfully in their preferred habitats and should experience poorer food intake rates elsewhere. These same predictions hold if species distributions are shaped by exploitative competition. We tested the foraging success hypothesis and ex- ploitative competition hypothesis as explanations of the habitat distributions of Sage Spar- rows (Amphispiza belli), Black-throated Sparrows (A. bilineata), and Dark-eyed Juncos (Junco hyemalis), which spend the winter in different habitats along an elevational gradient in the Sonoran Desert of southern California. Individuals of each species were transplanted between habitats in a portable aviary and observed while foraging on naturally occurring seeds. Predictions of the foraging success hypothesis and exploitative competition hypoth- esis were not confirmed. Only Dark-eyed Juncos achieved their highest foraging success in their preferred habitat. Black-throated Sparrows experienced nearly identical foraging success in all three habitats, yet are confined to only one of them naturally. Sage Sparrows foraged more successfully in habitats other than that in which they naturally occur. We conclude that immediate foraging gains do not explain habitat distributions of these birds, and that interference competition, predation, or other unknown factors are responsible. We contrast this finding with that seen in Gal'apagos finches in which bird distributions closely matched food supply.

Through plasticity in traits controlling clonal morphology, clonal plants possess the potential to selectively place ramets within a heterogeneous environment or "forage." Although many studies document plant responses that are consistent with foraging, few studies test directly whether plants can preferentially locate "good" patches or avoid "bad " patches when grown in a heterogeneous environment. We conducted such a test for Hydrocotyle bonariensis, a clonal dune species that inhabits soils known to be extremely patchy on both time and space. We subjected Hydrocotyle to a temporally constant but spatially variable soil resource environment created by clipped patches of grass. Clonal morphology on the patch treatment was compared to that in two homogeneous treatments: no grass and full grass. In order to predict the effect of our treatments on the long—term expansion of clones, we calibrated diffusion models of clonal growth with data on the morphological response of Hydrocotyle to our treatments. Clones of Hydrocotyle bonariensis were able to respond to the presence of patches by selectively placing ramets outside of grass patches, thus providing direct evidence of an effective foraging response. For each of the three traits identified as potentially contributing to an overall foraging response (branching, internode distance, and direction of rhizome growth), the response in and out of grass patches was substantially different from the degree of plasticity manifested by the two homogeneous treatments. For example, no difference in main rhizome internode lengths was found between the two homogeneous treatments. In contrast, ramets in the heterogeneous (patchy grass) treatment responded to their local environment by increasing internode lengths when in the unfavorable (grass patch) portion of the environment. Empirically calculated diffusion models indicate that habitat complexity has considerable impact on the long—term expansion of clones: only in the heterogeneous treatment did clones expand less rapidly in favorable habitat than they did in unfavorable habitat. In the heterogeneous treatment, Hydrocotyle rhizomes exhibited a previously undocumented behavior: they appeared to veer away from patches of grass. Finally, we discuss how the foraging response of Hydrocotyle ramets may be enhanced by its previously documented capacity for resource integration.

Tropical hummingbirds often forage in mixtures of several plant species. As a result, pollen is often transferred among heterospecific flowers. The "Sexual Architecture Hypothesis," a paradigm of pollination ecology and angiosperm evolution, suggests that the severity of such interspecific pollen transfer should reflect the similarity in the archi- tecture of sexual structures (specifically, placement of stigma and anthers) among the flowers involved. We evaluated the hypothesis with six species of Costa Rican cloud-forest plants varying greatly in sexual architecture but all pollinated by the same hummingbird, Lam- pornis calolaema. Captive Lampornis visited five long-styled, "pollen-donor" flowers of distylous, self-incompatible Palicourea lasiorrachis (Rubiaceae), then 20 short-styled re- cipient flowers of Palicourea, with or without five flowers of a second species intervening. We assessed intraspecific pollen transfer from Palicourea donors by counting pollen tubes in styles of the recipients. We examined interspecific pollen transfer between Palicourea and each of the five. other species by counting heterospecific grains deposited on stigmas of Palicourea recipients, and Palicourea grains deposited on stigmas and anthers of the heterospecific flowers. In a separate experiment, we examined how mixed-species flower visits modified the pollen loads actually carried by birds. Results did not uphold the Sexual Architecture Hypothesis. A single visit to any hetero- specific flower greatly affected the hummingbird's prior load of Palicourea pollen, re- gardless of where the second species placed its own pollen on the bird. Interspecific pollen transfer occurred consistently, but (1) heterospecific grains usurped little space on stigmas of Palicourea recipients and had no discernible effect on pollen-tube growth, whereas (2) few Palicourea grains landed specifically on anthers and stigmas of heterospecific flowers. Instead, numerous Palicourea grains apparently were scraped off on other floral structures such as the corolla. Consequently, every species of intervening flowers, regardless of its sexual architecture per se, strongly reduced pollen transfer from donor to recipient Pali- courea flowers, on average by 76%. In fact, the most drastic reductions resulted from the two species whose sexual architectures least resembled that of Palicourea. Results imply that "pollen scraping" by floral structures other than anthers and stigmas may be important and widespread among some animal-pollinated flowers. Thus, except in cases where no other floral structures contact pollinators, the positions of anthers and stigmas alone may have little bearing on the severity of interspecific pollen transfer and competitive interactions among plants sharing pollinators.

The purpose of this investigation was to accomplish a simultaneous eval- uation of both the costs and benefits of mixed-species foraging for a socially subordinate species. Specifically, we studied Carolina Chickadees that commonly forage in winter with a socially dominant congener, the Tufted Titmouse. We prevented these species from foraging together by removing titmice from five central Ohio woodlots. We then compared the biology of chickadees on these sites to that of chickadees on four sites where titmice had not been removed. Where titmice had been removed, chickadees foraged in more titmouse-like locations. Although this shift was statistically significant for two of the five niche parameters measured, there was a consistent pattern of shifts toward the titmouse niche in all five. We were unable to detect a change in chickadee vigilance in the absence of titmice. Chickadees showed statistically greater induced feather growth in the absence of titmice, indicating better nutritional condition, but we detected no differences in survival between treatment groups. We conclude that the foraging niche results provide evidence of a cost to chickadees of foraging with titmice. Furthermore, based on the feather growth results, we conclude that the costs and benefits for chickadees of this interaction sum to a net cost in foraging efficiency if not in survivorship.

The phenological relationships between nesting behavior of a specialist, solitary bee, Dieunomia triangulifera, and the flowering of its primary pollen source, Helianthus annuus, were studied for 3 yr at a site in northeastern Kansas, which contained between 50 000 and >150 000 nests. Activity patterns of D. triangulifera are closely synchronized with pollen availability on nearby sunflower plants in three ways: (1) each year, D. triangulifera became active within days of the beginning of the local sunflower bloom, and the emergence schedule of the entire population at the nest site was timed such that all females had constructed nests and were collecting pollen at peak bloom; (2) over a season the greatest numbers of provisioning females were active at peak bloom, when pollen was most abundant; and (3) over the day, females made more trips, each of which took less time when pollen was most abundant on nearby flowers than when it was scarce. Between seasons, female reproductive success varied positively with the availability of H. annuus pollen. When pollen was abundant, and the population at the nest site was low, °3 times more cells were provisioned than the following year when the bee population was high and pollen abundance declined. Furthermore, 20 pollen—collecting bee species (10 specialists and 10 generalists), besides D. triangulifera, used an estimated 68% of the available sunflower pollen at this site. These data suggest that resource limitation is important in shaping patterns of provisioning behavior in D. triangulifera. Other behaviors presumably related to reproductive success were variable and suggest resources were not limiting: (1) all females did not forage on each day, even at peak bloom; (2) variance in both trip durations and the numbers of trips made per day was large; and (3) females produced far fewer eggs in their lifetime than apparently possible. We suggest that the apparently conflicting evidence for resource limitation may be explained by a physiological limit on the rate of egg production; in such a case, the effects of resource limitation, while clearly important, might not be the primary factor limiting reproductive output. Female D. triangulifera benefit by being synchronized with their resource. Daily and seasonal synchrony enable females to collect a greater amount of pollen during shorter foraging trips. The translates into higher mean reproductive success and, at the population level, allows more females to be supported by the pollen produced from nearby plants. In short, those females that are synchronized with the sunflower bloom effectively increase their own resource availability. This phenomenon may be general, especially in short—lived organisms, i.e ., the evolution of specialization may be associated with fitness benefits based on synchrony with a preferred food source.

The predator-prey shell game predicts random movement of prey across the landscape, whereas the behavioral response race and landscape of fear models predict that there should be a negative relationship between the spatial distribution of a predator and its behaviorally active prey. Additionally, prey have imperfect information on the whereabouts of their predator, which the predator should incorporate in its patch use strategy. I used a one-predator-one-prey system, puma (Puma concolor)-mule deer (Odocoileus hemionus) to test the following predictions regarding predator-prey distribution and patch use by the predator. (1) Pumas will spend more time in high prey risk/low prey use habitat types, while deer will spend their time in low-risk habitats. Pumas should (2) select large forage patches more often, (3) remain in large patches longer, and (4) revisit individual large patches more often than individual smaller ones. I tested these predictions with an extensive telemetry data set collected over 16 years in a study area of patchy forested habitat. When active, pumas spent significantly less time in open areas of low intrinsic predation risk than did deer. Pumas used large patches more than expected, revisited individual large patches significantly more often than smaller ones, and stayed significantly longer in larger patches than in smaller ones. The results supported the prediction of a negative relationship in the spatial distribution of a predator and its prey and indicated that the predator is incorporating the prey's imperfect information about its presence. These results indicate a behavioral complexity on the landscape scale that can have far-reaching impacts on predator-prey interactions.

Body size is an important determinant of the diving and foraging ability in air-breathing marine vertebrate predators. Satellite-linked dive recorders were used during 2003-2004 to investigate the foraging behavior of 22 male California sea lions (Zalophus californianus, a large, sexually dimorphic otariid) and to evaluate the extent to which body size explained variation among individuals and foraging strategies. Multivariate analyses were used to reduce the number of behavioral variables used to characterize foraging strategies (principal component analysis, PCA), to identify individually based foraging strategies in multidimensional space (hierarchical cluster analysis), and to classify each individual into a cluster or foraging strategy (discriminant analysis). Approximately 81.1% of the variation in diving behavior among individuals was explained by three factors: diving patterns (PC1), foraging effort (PC2), and behavior at the surface (PC3). Individuals were classified into three distinct groups based on their diving behavior (shallow, mixed depth, and deeper divers), and jackknife resampling of the data resulted in correct group assignment 86% of the time. Body size as an independent variable was positively related to dive duration and time spent ashore and negatively related to time at sea, and it was a key parameter in PC2 used to classify the three distinct clusters. Differences among individual-based foraging strategies probably were driven by differences in body size, which enabled larger animals to dive deeper and forage more efficiently by targeting different and perhaps larger prey items. The occurrence of foraging specializations within a species and age class has implications for quantitative modeling of population-level predator-prey interactions and ecosystem structure.

This study examines flower choice by two syntopic butterfly species, Agraulis vanillae (Nymphalidae) and Phoebis sennae (Pieridae), and the energy consequences of foraging selectivity. Foraging selectivity is influenced by proboscis of P. sennae is °~1 cm longer than that of A. vanillae, it is able to forage at long—corolla flowers (three Ipomoea spp.) inaccessible to A. vanillae. Relative to other nectar sources exploited by the two species, these Impomoea species are highly rewarding energetically. Phoebis sennae increases its energy intake relative to A. vanillae by exercising greater discrimination between high— and low—reward nectar sources, avoiding several species with small nectar volumes that are avidly visited by A. vanillae. Consequently, the average foraging profit of P. sennae is nearly 4 times that by A. vanillae. These differences in foraging selectivity energy intake are closely related to energy storage and depletion rates, measured as age—related changes in gross lipid contents of adult butterflies. Adult activities in the less selective species, A. vanillae, are fueled to a greater degree with lipid reserves set aside at metamorphosis; A. vanillae emerge from pupae with 13—16% of dry mass allocated to lipids, and these lipid stores decline at a variable rate throughout adult life. By contrast, in P. sennae lipid accumulated adult feeding provides the largest proportion of energy stores. P. sennae emerge from the pupa with only 6% of the dry mass allocated to lipids, and accumulate lipid stores to levels as high as 40% of dry mass through most of adult life, depleting these lipid reserves only in late adulthood. Further, A. vanillae show significant sexual dimorphism in lipid content throughout life, with females containing greater quantities, while P. sennae show no such dimorphism. These results are discussed with respect to Boggs' model concerning the allocation of larval—derived resources to reproduction.

Food limitation has been thought to explain the evolution of single-chick broods in pelagic seabirds. We compared the meal sizes, feeding rates, and time budgets of Masked Boobies (Sula dactylatra), typical pelagic seabirds, which raise one chick per breeding attempt, with those of sympatric Blue-footed Boobies (S. nebouxii), which often raise two or more chicks to fledging. Breeding adult Masked Boobies forage farther from land than do Blue-footed Boobies. If longer transit times caused Masked Boobies to deliver less food per day than did Blue-footed Boobies, and thus limited brood size, we expected Blue-footed Boobies, but not Masked Boobies, to be able to provision a two-chick brood. Analysis of chick meal size and adult foraging effort (time spent foraging) indicated that Blue-footed Booby parents could typically feed two, but not three, chicks; brood size thus matches feeding capacity in this species. Masked Boobies, however, appear to have sufficient excess foraging capacity to raise an additional chick, yet do not. The different brood sizes of these species are frequently cited to support the idea that foraging biology determines brood size in seabirds. In contrast, we conclude that Masked and Blue-footed Boobies have similar capacities to provision two-chick broods; some other factor limits Masked Boobies, but not Blue-footed Boobies, to fledging single chicks. Obligate siblicide clearly limits brood size at the proximate level in Masked, but not Blue-footed Boobies. A role for sibling competition also seems likely at the ultimate level, either through an interaction with delivery frequency or by driving genetic parent-offspring conflict over brood size.

Ecological and behavioral strategies that reduce predation were examined in North American porcupines (Erethizon dorsatum) by testing two hypotheses: (1) habitat use varies with activity level and age class of animals, and (2) with increasing nutritional stress, individuals accept increased predation risk to avoid starvation. Observational and experimental data were gathered over a 2-yr period for a Great Basin Desert porcupine population during winter. Field experiments included manipulations of perceived risk using predator urines and simulated predator encounters with a dog (Canisfamiliaris). Compared to foraging animals, sleeping porcupines of all age classes avoided high risk (i.e., open) areas. Mass loss varied by habitat with individuals foraging in open areas (high risk) experiencing less mass loss than those foraging in groves (low risk). Age class also affected habitat use by foraging porcupines. Juveniles invariably remained in low-risk areas, even when more risky habitats contained potentially better quality forage. Two-year-old por- cupines shifted to high-risk areas as body condition deteriorated, but condition exerted little influence on habitat use by adults, as this age class utilized habitat similarly throughout winter. These data reveal how a species with formidable antipredator morphology uses habitats in risk-sensitive ways.

Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in struc- turally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology-microhabitat association is that species differing in morphology also differ in vulnerability to predators, that micro- habitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Homed Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal het- eromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat special- izations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.

We used a series of fenced exclosures to examine whether herbivory by mammals affected the structure and dynamics of an old-field plant community in Virginia. Early in the 3rd yr of the study purple clover, Trifolium pratense, a dominant member of the plant community, occurred at higher densities where herbivores were excluded relative to control plots. Following a 1-mo time lag, Trifolium became infected by the rust-forming fungal pathogen Uromyces trifolii; the incidence of infection was higher in exclosures relative to control plots where herbivores were present. Early-to-late season changes in the abun- dance of Trifolium were related to the degree of fungal infection and not herbivore presence/ absence. Data from the following year (when the fungus and Trifolium were rare) showed clover abundance to be higher in control plots than in enclosures. These results suggest that the negative influence of the fungus not only persisted through time, but more strongly affected host plant abundance in a 2-mo period than did herbivory that occurred in both years. While the direct effect of herbivory on food plants is negative, by keeping plant abundance below that at which fungal infections become epidemic, the net effect of her- bivores on some forage plants may be positive. The existence of strong indirect interactions is further evidence that the emphasis in community ecology (and in plant succession) should be on interactive networks rather than on species pairs.

When reproduction competes with the amount of resources available for survival during an unpredictable nonbreeding season, individuals should adopt a risk-sensitive regulation of their reproductive allocation. We tested this hypothesis on female reindeer (Rangifer tarandus), which face a trade-off between reproduction and acquisition of body reserves during spring and summer, with autumn body mass functioning as insurance against stochastic winter climatic severity. The study was conducted in a population consisting of two herds: one that received supplementary winter feeding for four years while the other utilized natural pastures. The females receiving additional forage allocated more to their calves. Experimental translocation of females between the herds was conducted to simulate two contrasting rapid alterations of winter conditions. When females receiving supplementary feeding were moved to natural pastures, they promptly reduced their reproductive allocation the following summer. However, when winter conditions were improved, females were reluctant to increase their reproductive allocation. This asymmetric response to improved vs. reduced winter conditions is consistent with a risk-averse adjustment in reproductive allocation. The ability of individuals to track their environment and the concordant risk-sensitive adjustment of reproductive allocation may render subarctic reindeer more resilient to climate change than previously supposed.

Sexual segregation in ungulates has important conservation and theoretical implications, but despite numerous studies, the impetus for this behavioral pattern remains a topic of debate. Sexual segregation hypotheses can be broadly grouped into social and ecological explanations, but only ecological explanations can adequately describe why sexes use different areas and habitats. The reproductive strategy hypothesis (RSH) and forage selection hypothesis (FSH) are leading ecological explanations, and although both have received support in the literature, neither the collective basis for that support nor overlap between these hypotheses has been adequately evaluated. This review analyzed seasonal sex comparisons of habitat forage quantity (n = 66), quality (n = 67), and diet (n = 63) from peer-reviewed studies of north temperate ruminants to test predictions of each hypothesis. Empirical data supported predictions of the RSH, but did not support two of three key predictions of the FSH. Males used habitats with greater forage quantity significantly more than females. But, contrary to predictions of the FSH, females did not use habitats with superior forage quality nor consume higher-quality diets more than males. Sexes typically used habitats and consumed diets of similar quality, and when differences were reported, males used higher-quality habitats significantly more than females. Results refute FSH arguments that differences in dietary requirements associated with sexual dimorphism are a universal explanation for sexual segregation in ungulates, but the physiological mechanism on which the FSH is predicated may explain why males consume poorer-quality diets when high-quality forage is scarce. The FSH, therefore, operates at a proximate level because it explains diet and habitat selection by males under certain environmental conditions, but multiple environmental factors may influence sexual segregation, and no single proximate explanation adequately describes this behavioral pattern. The RSH explains sexual segregation as the evolutionary response to differences in reproductive strategies: males choose habitats to maximize energy gains in preparation for rut, and females select habitats with combinations of resources that contribute to offspring survival. Consequently, the RSH provides an ultimate explanation that can be used to explain and interpret studies of sexual segregation in ungulates.

We examine environmental constraints on life history characters among three elevationally distinct populations of the desert lizard Sceloporus merriami in west Texas. We show how environmental gradients in temperature and food abundance interact to constrain body temperatures, daily activity times, growth rates, and age-specific body size. We suggest that these differences resulted in opposite responses from males and females with respect to their size and age at first reproduction. The highest elevation site, Maple Canyon (1609 m), has more rainfall, higher food availability, and cooler temperatures compared with the lower elevation sites, Grapevine ( 1036 m) and Boquillas (560 m). Lizards were active throughout the day at Maple Canyon, but were restricted to midday inactivity at the lower sites. Body temperatures (Tb) were higher during inactivity and late-afternoon activity at the lower sites. We suggest that low food availability, fewer hours to forage, and higher Tb constrained the energy budgets of lizards at Boquillas. Consequently, these lizards had low activity rates and low daily growth rates. Paradoxically, lizards at Maple Canyon, the most food-rich site, also exhibited low daily growth rates. This may have resulted from high daily activity expenditure (longer activity period) and lower food processing rate (low inactive Tb). Grapevine lizards had the highest individual activity and growth rates. On a seasonal basis, the pattern of activity time was reversed, and Maple Canyon lizards had a shorter annual active season by 1 mo compared with Grapevine and 2 mo compared with Boquillas. Despite low growth rates at Boquillas, a 2-mo longer activity season proxi- mally resulted in the largest yearlings by May-June. Maple Canyon yearlings grew at a similar rate, but over a shorter time period and were the smallest. At Grapevine, high summer growth rates resulted in large yearlings by late summer and the largest adults, whereas low summer growth rates at Boquillas and Maple Canyon led to smaller adults. The resulting difference in body size between yearling and older age classes was great at Maple Canyon and at Grapevine, which forced yearling males to delay reproduction until they had grown to a size to compete successfully for breeding territories. However, Boquillas yearlings and age > 1 yr lizards were much more similar in size, and many yearlings were able to acquire breeding territories. This suggests that male age of first reproduction was lowest at the low elevation site. In contrast, more yearling females reproduced earlier at the highest elevation site than at other sites. At Grapevine and Boquillas, females similar in size to breeding females at Maple Canyon did not breed in May-June and instead grew and were reproductively active in July-August. We hypothesize that the brief growing season at Maple Canyon may have constrained females to breed early to allow sufficient time for themselves, as well as for their offspring, to amass winter stores. Also, this would increase the likelihood that their offspring were large enough to breed as yearlings. Results suggest an interaction between resource levels and biophysical constraints that may greatly influence differences among populations in important life history character- istics. Although these responses are hypothesized to be proximally induced by environ- mental constraints, the resulting life history differences in age-specific resource allocation to growth, storage, and reproduction may significantly affect fitness.

Stems of the parasitic plant Cuscuta subinclusa (Convolvulaceae) discrim- inate among host species and invest in resource acquisition (coil around the stem of a host plant) relative to host quality in a way predicted by a marginal value model of resource use. C. subinclusa can be said to forage in that: (1) stem coiling, the necessary antecedent and determinant of resource uptake, precedes exploitation of host materials; and (2) mean coiling on a host species is proportional to mean reward per unit of investment (measured as biomass accumulation per unit length of stem in coil). Preliminary evidence indicates that coiling in C. subinclusa is induced by host bark flavonoids. Larger parasites are more likely to overwinter and set seed a second season than smaller ones, and parasites that start from overwintered tissue are significantly larger at flowering than are those that have started from seed. Thus, seed set is correlated with parasite size at the end of the season, linking the foraging response and fitness of the plant.

Two species of salamander, Plethodon cinereus, and Plethodon hoffmani were used in experiments designed to test the hypothesis that food is the object of competition. Plethodon hoffmani occurs in the Appalachian Ridge and Valley Province where it appears to exclude the closely related and contiguously allopatric P. cinereus. Detailed field observa- tions and a laboratory experiment confirm Jaeger's (1972) hypothesis that the food of sala- manders could be limited in availability during dry weather and suggest that the cause of the low availability is the reduced mobility of the salamanders rather than a reduced food supply. However, because stomach content data and behavioral observations indicate that salamanders do not forage when the surface is dry, it was reasoned that food competition could occur under only two restricted conditions: (1) when more than one animal is isolated under damp refugia, such as rocks and logs, during dry weather, and (2) when many animals emerge onto the surface to forage when wet weather follows a prolonged dry period. Empirical evidence suggests that salamanders do not clump in refugia as the habitat dries out and that they do not surface simultaneously to forage after a dry period. Thus, even though food is periodically unavailable, the staggered feeding schedule prevents the density of salamanders from ever reaching competitive levels. Interference mechanisms are not ruled out as mecha- nisms of the observed spacing patterns. Competition for space is suggested as an alternative to the food competition hypothesis.

Our previous observations and short-term experiments on two granivorous ant species in the California desert indicated that established colonies may compete in- traspecifically. Here we report the results of 3-yr neighbor-removal and food-addition experiments that tested for the presence of intraspecific and interspecific competition be- tween colonies. We followed the responses of territory use, diet breadth, foraging activity, and reproductive output (alate production) to the treatments. Both territory use and alate production patterns were consistent with the predictions of intraspecific resource compe- tition. We found no evidence for interspecific effects between colonies. Specifically, Ve- romessor pergandi "neighbor-removed" colonies foraged randomly throughout the treat- ments, while control colonies tended to forage away from their nearest nonspecific neighbor when resource levels were low. Both food-supplemented and neighbor-removed V. per- gandei colonies produced proportionately more alates than control colonies. There were no treatment effects on diet breadth or foraging activity at V. pergandei or Pogonomyrmex californicus colonies. Since other experiments on desert granivores have shown significant competitive effects on colony density over the same time scale, the lack of consistent effects of competition was most likely due to: (1) differences between the communities in relation to seed storage and productivity and (2) the fact that our experiments only tested for competition between established colonies and did not consider potential competitive effects of established colonies on alate survival (colony regeneration).

Rufous Hummingbirds in the Sierra Nevada Mountains were observed to forage in the edge areas of their territories during the first few hours of the day, evenly over their territories during midday, and in the territory cores in late afternoon. This distribution of foraging effort appeared to be inconsistent with maximizing their rates of net energy gain during foraging. If Rufous Hummingbirds were to maximize their net energy gain during individual foraging bouts, foraging effort would have to be distributed evenly over the territory at all times of the day. We present a model which suggests that the observed feeding behavior of territorial Rufous Hummingbirds leads to higher daily net energy gains and lower food losses than if the birds maximized only net energy gain during each foraging bout during the day. Intrusion pressure accounts for the early-morning bias of foraging to edge areas. Intruders attempted to rob nectar from flowers in the edge areas of territories much more frequently than in the cores, and a higher proportion of these attempts were successful. Intruders frequently returned within 5-10 min to the sites that they had just robbed, and returned most frequently to those territories where they were able to rob for the longest time (on average) before being detected and chased. Nectar levels on territories were highest at dawn, and since edges were more prone to food losses than were cores, the territory owner minimized losses by feeding in the edge areas early in the morning. After chasing intruders from their territories, owners often returned and fed in the site just robbed by the intruder, which also reduced food losses by depleting nectar levels in areas prone to intrusion. According to the model, reduction of reintrusion rates by lowering the yield per intrusion and thereby discouraging robbers contributes more to the owner's benefit than does simple pre-emption of the resource by the owner. Thus, the peripheral foraging behavior of hummingbirds and their feeding response following intrusions are components of territory defense. Such "defense by exploitation" may be common when food resources are being defended, and should be considered along with aggressive exclusion of intruders ("defense by interference") in the overall strategies of territorial animals. We close with a speculation that the late-afternoon foraging bias to the core may raise foraging efficiency during the time of day when the birds are heaviest and therefore have the highest foraging cost.

In an abandoned field near Ithaca, New York, bee species partitioned the available flower resources, primarily by foraging at different times of the season and by visiting different flower species. Honey bees (Apis mellifera) concentrated on different flower species than did native bees. In spring, most flowers bloomed along roadsides, in cultivated fields, or in forests. Many were clustered in spatial distribution and had short blooming times. Spring—flying native bees included many species adapted to forage on native flower species that bloom before the tree canopy closes. Apis mellifera, an introduced species, appeared to outcompete native bees at large clusters of attractive flower species such as apple and the crucifer Barbarea vulgaris. Blooming of introduced herbs increased in the field in early summer. This period of resource abundance was exploited primarily by primitively eusocial halictine bees. The multivoltine seasonal cycle of these bees allowed them to build populations over the season and to forage in large numbers in early and mid summer. Halictus ligatus was the most common early summer halictine. This species concentrated its foraging on different flowers than did other early summer bees, probably because of differences in innate flower preferences. A profuse bloom of native goldenrods in late summer was the seasonal peak of resource abundance. Honey bees partitioned goldenrod species with late summer native bees on the basis of flowering time. Native bees must complete their seasonal cycles and enter diapause before the killing frost. They therefore foraged on early—blooming goldenrod species such as Solidago juncea. In contrast, honey bees overwinter as active bees within their hives. They were most common on later—blooming goldenrods, where they collected much of the nectar required for overwintering. Competition among foraging bees was probably most important in the spring at this site. Partitioning of flower species and differences in seasonal flight times resulted primarily from differences in innate flower preferences, number of generations per season, and means of overwintering.

Differences in microhabitat utilization by sparrows wintering in southeastern Arizona, USA may be sufficient to explain their coexistence. The species either forage in different mac- rohabitats or in the same habitat at different distances from tree or shrub cover. Seed size partitioning within habitats is sufficient to explain coexistence only in one case where the seed size distribution is bimodal. The species that forage in plains grassland habitat differ by foraging in concentric rings about tree or shrub cover. The behavior of each of these species appears to enhance predator avoidance and these behaviors change in a regular pattern with distance to cover both within and between species. Species foraging close to cover are social and tend to be conspicuous both in behavior and morphol- ogy. Two different strategies adapt species to avoiding predators at great distances to cover. In dense grassland, with increasing distance to shrub or tree cover, species in the genus Ammodramus are increasingly solitary and cryptic. In sparse grassland, Chestnut-collared Longspurs are social and cryptic on the ground but conspicuous in flight.

Asynchronous ripening of fruits within individual, temperate, summer-fruit- ing, bird-dispersed plants has been proposed to be an adaptation to avoid satiation of dispersers when they are not abundant. I tested a prediction of this hypothesis by manip- ulating ripening synchrony on individual Amelanchier arborea trees and comparing ripe fruit removal rates from these trees to paired controls. Artificially synchronous ripe fruit displays were created by cutting off unripe fruit and replacing them with ripe fruit. Fruit removal was faster for the manipulated, synchronous tree in three pairs, slower in two pairs, and not significantly different in five other pairs, suggesting that synchronous displays did not satiate dispersers. This unexpected result is attributed to violation of the unstated assumption that dispersers are relatively sedentary in their foraging in the summer and do not concentrate at large fruit displays. In this study the major frugivore of A. arborea, the Cedar Waxwing (Bombycilla cedrorum), foraged widely in flocks of 2-6; other reports reveal that some other frugivorous bird species also forage widely in the summer. This nonter- ritorial foraging implies that the arrival of dispersers is temporally unpredictable from the perspective of each plant, and it is this temporal unpredictability that may favor asyn- chronous ripening in plants lacking adaptations for ripe fruit persistence.

In an aviary, field—captured Black—capped Chickadees, Parus atricapillus, learned to forage preferentially at trees with either artificially or caterpillar—damaged leaves. The birds also distinguished between species of broad—leaved trees. These individuals showed differences in foraging behavior, possibly related to previous conditioning in the field. When captured, two of the birds were already using leaf morphology (rolls and discoloration) to search for caterpillars. Two others initially disregarded leaf morphology and instead scanned for prey directly. However, when the potential prey was highly cryptic or hidden, the birds all learned to use both tree species and leaf damage simultaneously as cues in their searching effort for specific prey. Caterpillars that were palatable to the chickadees had feeding behaviors that tended to minimize their apparent feeding damage on leaves, while caterpillars that were not eaten did not have these behaviors. See full-text article at JSTOR

Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Ne- braska, USA, nest in colonies that serve as information centers in which unsuccessful individuals locate and then follow successful individuals to aerial insect food resources. I investigated to what degree colony size affected the efficiencies at which Cliff Swallows foraged for food, and to what degree information transfer among colony residents might represent a benefit of living in colonies. The number of Cliff Swallows departing from a colony each hour to forage increased significantly with colony size, meaning that individuals did not wait as long to locate appropriate foraging associates in large colonies as in small colonies. Waiting intervals between an individual's arrival at its nest and its departure on its next foraging trip increased as the number of birds nesting in a colony declined. Parental Cliff Swallows in large colonies returned with food for their nestlings more often and brought more food per trip, than did parental Cliff Swallows in small colonies. As a result, nestling body mass at 10 d of age increased significantly with colony size, when the confounding negative effects of ectoparasites on nestling body mass were removed by fumigating nests. Adult Cliff Swallow body mass late in the nesting season during the period of feeding of nestlings increased significantly with colony size even in the presence of blood-sucking ectoparasites (in nonfumigated colonies). Experimental reduction of large Cliff Swallow colonies to the approximate size of small colonies suggested that nestling and adult body mass in these colonies did not vary with colony location, meaning that enhanced foraging efficiency of birds in larger colonies was probably attributable to more efficient transfer of information among the larger number of colony residents and was probably not attributable to differences in local resource characteristics near colonies of different sizes. Enhanced foraging efficiency through information centers is likely a major benefit of coloniality for Cliff Swallows.

In coniferous forests of central Sweden, tits (Paridae) and the Goldcrest, Regulus regulus, exploit nonrenewable resources in their group territories during winter. The smaller and socially subordinate species, the Goldcrest and the Coal Tit, Parus ater, forage on the outermost tree parts, while the larger and dominant Willow Tit, Parus montanus, and Crested Tit, Parus cristatus, forage on the inner tree parts. We removed Coal Tits and Goldcrests in three flocks in early winter to see if their absence would cause changes in the foraging patterns of the two dominant species. In late winter, Crested Tits foraged farther outward on branches of spruce in experimental flocks than they did in the control flocks. In spruce, Willow Tits foraged nearer the trunk than Crested Tits, and they did not respond to the experiment. In pine, Willow Tits, however, did move from branches to twigs in the absence of Coal Tits and Goldcrests. The experiment indicates that ex- ploitation competition directly based on food depletion, without any interference, may influence the use of foraging sites by tits in coniferous forests.

The relationship between the pollination biology of a tropical plant, Bauhinia pauletia, and the foraging strategies of the nectarivorous bats visiting it was studied. At least two bat species are pollen vectors, Plyllostomus discolor and Glossophaga soricina. Artibeus jamaicensis and Sturnira lilium were also captured near Bauhinia flowers. Larger bats (P. discolor) drain flowers of nectar and forage in groups, while smaller bats (G. soricina) make brief visits and forage independently. These foraging strategies should optimize energetic gain for the bats and promote outcrossing for the plant. Bauhinia pauletia is self-compatible, but is found where conditions favor outcrossing. Andromonoecism (the presence of hermaphrodite and male flowers) in this species appears to be an adaptation to pollination by large pollinators that also promotes outcrossing.

Feeding strategies of the little brown bat, Myotis lucifugus, were investigated in southern New Hampshire USA from early May through late 1974. Nightly food consumption was estimated by comparing mean prefeeding body weights with postfeeding weights taken as individuals returned to the roost from their first feeding period (at 2200 to 2400h) and from a subsequent foraging period (at 0330 to 0500 h). Pregnant bats consumed an average of 2.5g of insects (2.72kJ/g prefeeding body weight) nightly. lactating females ate 3.7 g (4.23 kJ/g), and juveniles ingested 1.8 g (2.47 kJ/g). Increased food consumption in lactating bats accommodated reproductive energy demands and was facilitated by rising food availability. Increasing levels of independent food consumption in juveniles accompanied weaning. Fecal analysis revealed that diets of individual bats were diverse. All available insects 3 to 10 mm in body length were accepted as food items. Nematoceral Diptera were by far the most common insects taken in light—trap samples, and constituted a major portion of the diet throughout the summer. Coleoptera, Trichoptera, Lepidopter, Ephemeroptera, and Neuroptera were also consumed in appreciably numbers. Comparison of dietary composition with prey availability indicates that pregnant bats consumed 3—10 mm prey in approximate proportions encountered during June, when insect availability was low and unpredictable. However, lactating, postlactating, and nonproductive ♂ ♂ exhibited more selective feeding in July, when insects were more abundant. This increase in selectivity reflected exploitation of beetles and mayflies, which were uncommon in trap samples. In August, juveniles approximated random feeding patterns, as they learned to forage. We suggest that increased resource availability allowed selective feeding in adult bats during July, as predicted by presy selection models. However, reduced discriminatory abilities may prevent similar levels of prey selection in juveniles.

This report describes the foraging pattern of the Florida harvester ant Pogonomyrmex badius in a high-density population of colonies. The foraging pattern has both promoted and been influenced by the colony distribution. Pogonomyrmex badius forages from short trails which extend into a surrounding foraging range. Direction of foraging trails is influenced by the location of a colony's near neighbors. Seasonal nest relocations always occur along a foraging trail, usually the main trail. Foraging ranges are not actively defended, but are used almost exclusively by foragers from a single colony. Foraging ranges will be extended into an area abandoned by neighboring foragers, indicating that forager presence may define each colony's range. Colony distribution has remained essentially the same for several years, despite seasonal nest relocations and addition of new colonies. Establishment of trails and exclusive foraging ranges by each colony minimizes encounters with neighboring foragers and guarantees access to available resources; this pattern also promotes maintenance of the existing colony distribution and partitioning of resources.

I describe a study in which free—roaming downy Woodpeckers (Picoides pubescens) were allowed to forage in three different, patchy stochastic environments where patches (small, thin logs with holes drilled into them to hold food items) contained either zero or a fixed number of food items. All the patches in an environment were, however, identical in appearance. Downy Woodpeckers systematically searched the holes of a patch for food items, and thus the foraging task for an energy—maximizing woodpecker was to determine to what extent to sample a patch without success (without finding a food item) before giving the patch up as being empty and moving on. A model is presented to determine the optimal sampling solution. Although this foraging task is relatively simple, optimal solutions to many stochastic foraging problems may be quite complex. Thus, the observed foraging behavior is compared to the model's predictions as well as to some suggestions form the literature of simple behavioral mechanisms (e.g., a fixed giving—up—time strategy) that animals may use to approximate optimal solutions. The results show that the woodpeckers clearly recognized the fact that some patches contained food items while others did not, and use this information in following a sampling strategy more sophisticated than any of the suggested simple behavioral mechanisms. The predicted sampling behavior on empty patches and that observed were in qualitative but not quantitative agreement. The model predicted that a single number of holes in a patch should be sampled without success while, in fact, a distribution of number of holes sampled was observed. The mode of the distributions, however, corresponded to the predicted value in two of the three environments. It is shown that the woodpeckers did not know with complete accuracy at least some of the model's parameters, and that this may be related to a weak counting ability. A weak counting ability, the need to sample for environmental changes, a stochastic element in a bird's behavior itself, or some combination of these factor may contribute to the observed variability in sampling behavior. Nevertheless, perhaps through some ability to estimate time, the woodpeckers exhibited a sampling strategy very similar to that predicted by a simple energy maximization model, and such sampling abilities are crucial to more complex stochastic foraging models.

Males of four congeneric species of wood warblers (Parulidae) inhabiting a spruce forest of coastal Maine were found to forage higher than their females. They were the Magnolia (Dendroica Magnolia), Myrtle (D. coronata), Black—throat Green (D. virens), and Blackburnian (D. fusca) warblers. The foraging areas of males were nearer the height of their singing areas than were their nests, while foraging areas of females were nearer the height of their nest than the singing perches of their males. Very few differences were observed between sexes in the part of the tree utilized (proximal parts of limbs, tips of foliage, etc.) During the period of incubation, the foraging rates of females became noticeably greater than those of their males; before and after this period differences were less pronounced or non—existent. The partitioning of sexes results in a situation where males can forage readily while remaining maximally conspicuous, and females can forage in an area maximally adjacent to their nests. During t...

In the prairie region, the maintenance of conditions favorable for adequate production of grass is accomplished in nature by the development of biotic communities in equilibrium with the climate. In such communities, occasional overgrazing is perhaps common, and doubtless was present under primitive conditions, but continued overgrazing such as has been the result of poor range management by the white man has resulted in serious damage. Information necessary for correct methods of range management which will make possible permanent maintenance of favorable grassland conditions must come principally as a result of intensive range research. Since the removal of bison, rodents and lagomorphs are the principal wild grazing mammals on ranges and often consume large quantities of forage. They are especially harmful during droughts or under conditions of overgrazing. A range may be stocked within its carrying capacity with livestock but additional pressure by rodents may result in depletion (Vorhies and Taylor, '22). Therefore, information concerning rodents and their reactions to conditions following overgrazing seems necessary for proper range management. An attempt is made to supply quantitative data regarding the relative abundance of some common rodents and lagomorphs in the various types of grassland found in cental Oklahoma. The problem was suggested by Dr. A. 0. Weese of the University of Oklahoma under whose guidance the work was done.

Henslow's Sparrow (Ammospiza henslowii) is a secondary sexually monomorphic breeding bird in meadows, hayfields, prairies, grasslands, and some croplands in eastern North America. Foraging behavior during the nestling period involved no intersexual differences in prey selection, and both sexes were opportunistic in food choice. Sexual spatial partitioning of the feeding niche was accomplished by adults foraging in different directions and at different distances from the nests. Males, which defend the territories alone, forage at greater distances from the nest than females. Females forage closer to the nest and in different areas than males, which enhances survival of young because females do most of the feeding and all brooding of young. See full-text article at JSTOR