Search

Abstract Ecological factors and individual-specific traits affect parasite infestation in wild animals. Ixodid ticks are important ectoparasites of various vertebrate hosts, which include passerine bird species such as the great tit (Parus major). We studied various key ecological variables (breeding density, human disturbance) and phenotypic traits (exploratory behaviour, body condition) proposed to predict tick infestation probability and burden in great tits. Our study spanned 3 years and 12 nest box plots located in southern Germany. Breeding, adult great tits were assessed for exploration behaviour, body condition, and tick burden. Plots were open to human recreation; human disturbance was quantified in each plot as a recreation pressure index from biweekly nest box inspections. Infested individuals were repeatable in tick burden across years. These repeatable among-individual differences in tick burden were not attributable to exploration behaviour. However, faster explorers did have a higher infestation probability. Furthermore, body condition was negatively correlated to tick burden. Recreation pressure was correlated to increased infestation probability, although this relationship was just above the threshold of statistical significance. Our study implies that avian infestation probability and tick burden are each determined by distinct phenotypic traits and ecological factors. Our findings highlight the importance of animal behaviour and human disturbance in understanding variation in tick burden among avian hosts. Significance statement Various abiotic and biotic factors, including personality type, influence tick parasitism in birds, but exactly how all these factors interplay remains unclear. We studied a wild population of great tits over a 3-year period and assessed birds for their exploration behaviour and tick infestation. We found that more explorative great tits were more likely to be infested with ticks. By contrast, faster explorers did not have higher tick burdens. Tick burden was nevertheless moderately repeatable among individuals. Our results imply that animal personality influences the probability of parasite infestation, and that infestation likelihood versus intensity are determined by distinct mechanisms. Our work highlights the importance of animal behaviour to understand parasite infestation in wild populations.

Flushing distance (FD, the horizontal distance between a parent bird when it leaves its nest and an approaching predator) is one measure of nest defense and of risk-taking; parents that stay too long risk being killed, whereas those that flush too early risk at the very least impairing development of their young, and at the very worst leaving them unprotected against predators. Thus, FD should be under strong natural selection. A general prediction is that incubating birds will remain on a nest being approached by a predator until risks of staying reach a threshold that outweighs costs of fleeing. This threshold is predicted to vary depending on a brood’s value, parental characteristics, environmental conditions, and learning that repeated visits pose a limited or no threat. We evaluated FD in a nest box population of tree swallows (Tachycineta bicolor) relative to each of these. We obtained 246 FDs from 66 different nests over 2 years. We found some evidence that FD increased with clutch size (tendency), female age, and air temperature, and decreased with greater overhead vegetation density; six additional associations were not significant. Given the lackluster support for the predictions we tested, we did a review of the literature and similarly found limited support for most of the associations we tested despite the entrenched view that these relationships are commonplace. We submit that further insights are needed into understanding predictors of FD in incubating birds. Parent birds sitting on eggs are proverbial sitting ducks, although they do have the option of fleeing predators. However, parents may be willing to sacrifice themselves to protect their eggs if the latter become sufficiently valuable. One assumption is that eggs increase in value closer to hatch, and one prediction is that parents will be more reluctant to fly from their nests later in incubation. We tested this and other predictions in a population of nest-box-using tree swallows in eastern North America. The relatively weak support we obtained for our predictions suggests that we need to reevaluate our assumptions in this area of research.

Itishypothesizedthatvariation inimmunefunction between individuals is due to costs incurred to sustain it. Sup- port for this hypothesis mostly comes from short-term studies on the either costs of innate responses or a combination of innate and antibody responses. Key studies on the fitness and physiological costs of acquired immunity, in which the antibody response is specifically stimulated over a long peri- od, are lacking. We specifically stimulated the antibody re- sponse against a virus (Newcastle virus) in male European starlings (Sturnus vulgaris) for 2 months to test whether im- munization reduces a fitness-related trait (song rate) and in- creases oxidative stress. Immunization did not affect the total song rate, but it caused a reduction of the undirected song rate (produced away from the nest box and mostly used for estab- lishing dominance hierarchy). We also found that immunized birds had a lower nest-box-oriented song rate (mostly used to attract females) than control birds although the interaction between treatment and sampling period was not significant. Immunization did not cause any changes in the blood oxida- tive status. Starlings with a higher nest-box-oriented song rate had significantly lower levels of oxidative protein damage. Finally, starlings that skipped the antibody response had an oxidative status similar to that of starlings that produced anti- bodies, but they had overall a lower rate of undirected song. Our results suggest that (i) immunized starlings preserved the song used to attract mates but not that used in social interac- tions and (ii) the antibody response incurs costs that are reflected in the expression of song, but also that these costs are unlikely to be determined by oxidative stress. Our results also suggest that bird song might convey information about a male's oxidative status.

Recent studies indicate that polymorphisms of the DRD4 gene may be related to behavioural variation in mammals and birds. The purpose of this study was to ascertain a connection between DRD4 genotypes and behavioural patterns in a wild passerine bird during breeding time. We evaluated changes in birds’ parental provisioning behaviour in the presence of a novel object. As a behavioural trait, we measured the duration of feeding interruption from the first time that each bird noticed the novel object until they entered the nest box. We found a gender-specific association between DRD4 gene polymorphism and parental behaviour. Males with a CC genotype delayed feeding for a longer period than those with CT and TT genotypes. No significant effect of genotype on provisioning behaviour was observed among females. We conclude that (i) DRD4 gene polymorphism in wild birds can be associated with behaviour related to fearfulness, and (ii) this effect is supposedly gender-specific during the breeding time, potentially explained by sex differences in parental care or hormonal levels.

Parents may feed nestlings based on positional cues resulting from sibling competition for favored positions at the front of a nest cavity, or they may assess begging intensity regardless of location within the nest. We independently manipulated proximity to the nest entrance and hunger level to determine whether western bluebird (Sialia mexicana) parents favor hungrier nestlings or instead feed nestlings closest to the nest entrance. We used a balanced design to vary hungry and fed nestlings between the back and front of the nest box. We deprived half of the nestlings in each brood of food for an hour, and then, each nestling was constrained using Plexiglas dividers to a quadrant in the nest box. We videotaped inside the box to assess begging intensity and food delivery to individual nestlings. Hungry nestlings begged more intensely and received more food than fed nestlings, regardless of their proximity to the nest entrance. However, when delivering the first feed to nestlings, parents favored front nestlings over hungry nestlings, possibly due to a bias resulting from the manipulation, which involved replacing and rearranging nestlings after a 1-h period. Overall, parents did not favor nestlings in the front of the nest over those in the back. Parents therefore are able to distinguish hungrier nestlings and preferentially feed them under favorable environmental conditions, when brood reduction is uncommon. When parents choose which offspring to feed, they may assess begging signals, which can include movements and vocalizations that vary in intensity. Alternatively, cavity-nesting species may use simple rules such as favoring offspring that have pushed to the front of the nest. We conducted an experiment to determine whether western bluebird (Sialia mexicana) parents would preferentially feed hungry nestlings over fed nestlings when Plexiglas dividers prevented competition and equalized the frequency with which hungry and fed nestlings were in the front versus back of the nest. After depriving half of each brood of food for an hour, we measured begging intensity and food delivery. Parents preferentially fed hungrier nestlings, which begged more intensely, regardless of their position in the nest, indicating that parents are able to distinguish between hungry and fed nestlings.

Animals constantly need to acquire information about the environment for settlement decisions, either by using a trial-and-error strategy or by using public information by monitoring conspecifics. We studied a nest box population of Eurasian kestrels Falco tinnunculus in western Finland to test if pellets and other prey remains accumulated on the bottom of nest boxes are used as public information during settlement. During 2002–2013, nest boxes were randomly cleaned (treatment) or left un-cleaned (control) in each season. It is possible that kestrels reuse nest boxes which include information of successful nesting (i.e. have not been cleaned) because they indicate previous breeding attempt at the site. At the same time, this decision may entail costs because of blood-sucking ecto-parasites like Carnus hemapterus overwintering in the layer of pellets. First, we found that egg-laying date was significantly earlier in un-cleaned control boxes than in cleaned treatment boxes, indicating the use of public information revealed by pellets in the settlement decision. Second, the ecto-parasite burden of young nestlings (age 6–15 days) was significantly higher in un-cleaned control nest boxes. We found higher ecto-parasite infestation in early and lower infestation in late nests, a seasonal trend that is in disagreement with the ecto-parasite avoidance hypothesis. Contrary, in overall lower-infected cleaned boxes, ecto-parasite prevalence remained equal throughout the season. However, the ecto-parasite burden had no obvious effect on breeding success. We conclude that the use of pellets revealing successful breeding attempt of the previous year as public information appeared to be important in the settlement decision of kestrels.

This study addresses a question about the nest-site selection process of honeybee swarms: how do the scout bees know when to initiate the preparation for their swarm’s move to their new home? We tested the quorum-sensing hypothesis: that the scouts do this by noting when one of the potential nest sites under consideration is being visited by a sufficiently large number of scouts. A falsifiable prediction of this hypothesis is that delaying the formation of a quorum of scout bees at a swarm’s chosen nest cavity, while leaving the rest of the decision-making process undisturbed, should delay the start of worker piping (the prepare-for-takeoff signal) and thus the takeoff of the swarm. In paired trials, we presented each of four swarms once with five nest boxes close to each other at a site and once with a single nest box. The multiple nest boxes caused the scouts visiting the site to be dispersed among five identical nest cavities rather than concentrated at one. We observed long delays in the start of piping and the start of takeoff in the five-nest-box trials relative to the one-nest-box trials. These results provide strong support for the quorum-sensing hypothesis.

Nest predation has been suggested as an explanation of the adaptive significance and evolution of conspecific brood parasitism, an alternative reproductive tactic pursued by females in several animal taxa. I used new nest boxes that contained only decoy eggs and were erected on lakes differing in real nest predation risk to test this hypothesis in the common goldeneye (Bucephala clangula), a hole-nesting duck. I used broken eggs to simulate predation risk of the boxes to determine if parasites having no previous experience with the boxes discriminate between seemingly safe and risky nest sites. Parasites laid eggs in the experimental boxes independently of the simulated predation risk, suggesting that they do not use broken eggs or nest disarray as indicators of predation intensity. Parasites preferred experimental boxes on lakes where real nest predation risk was low, supporting the nest predation risk hypothesis. Assuming that females in high risk areas have had experience of nest predation, they may take this into account in selecting host nests.

Nestling birds produce a multicomponent begging display that has visual (e.g. posturing) and vocal (e.g. call rate) elements. Most work on the function of the display has focused on each component separately. However, understanding the evolution of complex displays such as begging requires knowledge of how the components function collectively. The purpose of our study was to determine how postural intensity and calling rate together influence parental feeding decisions in tree swallows, Tachycineta bicolor. We compared how begging components responded to a manipulation in which pairs of nestlings were either free to approach the parent when it arrived to feed (unconfined treatment) or confined to the back of their nestbox by a Plexiglass partition (confined treatment). We found no significant differences in postural intensity between treatments, but calling rate was significantly higher in the confined treatment. In both treatments, postural intensity, but not calling rate, correlated with hunger. Both components positively and independently correlated with the likelihood of a nestling being fed, although the correlation with postural intensity was stronger. Previous work suggested that both posture and call rate advertised hunger in nestling tree swallows. Here, call rate was not associated with hunger, but rather was affected by nestling position. These results suggest that calling may serve an additional role in helping nestlings in disadvantaged positions attract parental attention. The results also suggest that calling may have a complex relationship with hunger, position and nestmates.

A common criticism of nestbox studies is one of creating artificial nesting conditions and breeding behavior different from what would be seen under natural conditions. We assessed the frequency of extra-pair paternity (percentage of broods with at least one extra-pair young) in 25 families of tree swallows (Tachycineta bicolor) nesting in natural cavities and compared it to that in a nestbox population. We found that 84% of females nesting in natural cavities obtained fertilizations from extra-pair males. These extra-pair males fathered 69% of all nestlings. Studies of tree swallows breeding in nestboxes have shown that 50–87% of broods contained extra-pair young, with extra-pair males fathering 38–53% of all the young. In broods with extra-pair paternity, natural cavities contained a significantly greater proportion of extra-pair young than did nestboxes. Despite differences in nesting habitat and female age structure, the frequency of extra-pair paternity did not differ significantly between the natural-cavity and nestbox populations. Therefore, the presence of extra-pair paternity in tree swallows is not an artifact of nestboxes or of artificial nesting conditions.

Conflict between parents and offspring may result in offspring exaggerating their needs and parents devaluing their begging signals. To determine whether this occurs, it is first necessary to establish the link between need, begging and parental response. The purpose of our study was to examine these relationships in tree swallows (Tachycineta bicolor). Parents preferentially fed nestlings that begged sooner, reached higher and were closer to the front of the nestbox (Fig. 1). Begging intensity of both individuals and entire broods increased with relatively long periods between feeding visits. Within broods, parents responded to increased begging intensity by increasing their feeding rate, although this effect was relatively weak. Large and small nestlings did not differ in their begging behavior and all nestlings, regardless of size, were fed at similar rates. Despite the overall equity in feeding, male parents preferentially fed larger nestlings while female parents fed smaller nestlings. Nestlings did not increase their begging intensity in response to begging by nestmates. Our results suggest that begging is related to need in this species and that parents respond to variation in begging intensity.

In a Canadian population of tree swallows, Tachycineta bicolor, DNA fingerprinting has previously shown that half of all broods contain many offspring resulting from extra-pair copulations (EPCs), whereas the other half contain only legitimate offspring. This bimodal pattern of extra-pair paternity might be due to variation in the effectiveness of male paternity guards, variation in female ability to resist EPCs, and/or variation in female pursuit of EPCs. Here we report experimental evidence for female control of copulations and fertilizations and the occurrence of two alternative copulation strategies among females in this population. Ten paired male tree swallows were removed on the day their mates laid the first egg. Replacement males took over the nestbox within 0.5–23 h and attempted to copulate with the widowed female. Assuming that eggs were fertilized approximately 24 h prior to laying, the first two eggs were fertilized before the male was removed, while the third and subsequent eggs could potentially be fertilized by the replacement male. Fingerprinting revealed that the first two eggs were sired by the resident males in five nests and by extra-pair males in the remaining five nests. The widows that had been faithful to their initially chosen mate rejected copulation attempts by the replacement male until most of the eggs had been laid. Consequently, nearly all eggs laid by these females were sired by the original male. The widows that had been unfaithful prior to male removal copulated sooner with the replacement male than females that were faithful to their mate. However, these replacement males also had a very low fertilization success; most eggs were sired by males that were not associated with the nest. This is consistent with the situation in non-experimental nests where unfaithful females copulate with their mate at the same rate as faithful females, yet unfaithful females have a majority of offspring sired by extra-pair males. We conclude that fertilization patterns to a large extent are determined by the female through active selection and rejection of copulation partners, though our results also allow some speculation that females have control over sperm competition. Female copulation tactics are probably determined some currently unknown fitness benefits of having the offspring sired by particular males.

Hypotheses regarding evolution of polygyny were tested using prothonotary warblers (Protonotaria citrea), through experimental manipulations of habitat quality. Two experiments were performed. Experiment 1 involved supplying nest boxes in different breeding habitats: flooded riparian areas (high food availability) and adjacent dry bottomland (low food availability). Nine bigamous matings were induced in this experiment, all occurring in flooded habitats even though monogamous mating opportunities existed in dry areas. Costs of polygyny for secondary females in flooded habitat were similar to costs incurred by monogamous females in dry habitat. Thus, a polygyny threshold apparently existed, possibly based on differences in food abundance in the two habitats. However, male quality covaried with habitat quality, as males in flooded areas were older and larger than males in dry habitat. Secondary females may have chosen polygyny in the best breeding situation, a combination of male and territory quality. In Experiment 2 nest box density was varied within flooded habitat to allow males to monopolize different numbers of nest-sites. Monogamous females settled earlier on territories with a large number of nest-sites, and males that defended many nest-sites were more likely to become polygynous. Male physical characteristics were not related to occurrence of polygyny in flooded habitat. Limitation of suitable nest-holes ultimately constrains occurrence of polygyny in prothonotary warblers.

One explanation for polygyny in pied flycatchers (Ficedula hypoleuca) is the deception hypothesis, which proposes that females mate with already-mated males only because they are unaware that such males already have a mate on another territory. Recently this hypothesis has been criticized on the grounds that already-mated and unmated males differ sufficiently in their behavior that human observers can discriminate between the two classes fairly easily. Here we test whether male pied flycatchers change their behavior when visited by a female so as to make this discrimination more difficult. In our experiments we presented a caged female near the nestbox of an advertising male, in order to mimic the situation in which a female investigates the male and his nest site. During control periods, when only an empty cage was presented, we found that already-mated males were present on their secondary territories significantly less and sang significantly less than unmated males on their primary territories, confirming the earlier results. When we presented the stimulus female, all behaviors we measured changed significantly relative to control periods for both mated and unmated males: both classes of males increased the time spent on the territory, decreased singing rates, and increased various courtship behaviors. During the experimental periods there were no significant differences between mated and unmated males on any of the behavioral measures. Discriminant analysis was more successful in classifying males as to mating status using data from the control periods (without female) than using data from the experimental periods (with female). We conclude that when a female is present male pied flycatchers change their behavior in ways that make it more difficult to discriminate mated from unmated males.

This paper presents major new evidence for active mate choice of female pied flycatchers, Ficedula hypoleuca. Fifteen color-ringed females were released into a study area containing 23 unmated males defending one nestbox each. Through intensive surveillance, the behavior of the females was observed during 2 consecutive days. Twenty-two of the males received a total of 131 female visits. Six of the females settled in the study area, and their premating period lasted 1.3–2.5 days. The females were seen searching for mates for at least 6–32 h and were seen visiting at least 1–9 different males. Hence, some of the females rejected males before mating. Nevertheless, the females settled close to the site of release (range: 16–243 m), suggesting that they mated with one of the first males encountered. Females visited males most frequently in the morning, and the diurnal distribution of visits was significantly correlated with male song activity.

We tested the effect of manipulation of breeding density on the occurrence of extra-pair paternity in a blue tit (Parus caeruleus) population during two consecutive years. In a homogeneous oak forest, nest-box manipulation provided a high density plot (plot A, 1.10 and 1.32 pairs/ha) and a low density plot (plot B, 0.43 and 0.46 pairs/ha). Microsatellite analysis on 91 broods revealed a higher proportion of extra-pair paternity in broods in plot A (mean of 17.2%) than in plot B (mean of 11.4%). A correlative approach showed that the proportion of extra-pair young in broods was affected by the number of breeding neighbours within 100 m around the nest-box, by the distance to the nearest breeding neighbour and by an additional plot effect. However, the nearest neighbours accounted for only 39.3% of extra-pair paternities and distance to extra-pair fathers was significantly higher than the nearest neighbour distance in both plots. This implies that the effect of density on the occurrence of extra-pair paternities is associated with active female choice to enhance the brood fitness. Although there were more extra-pair young in broods when density was high, the number of extra-pair fathers did not increase and stayed close to one. We suggest that density increases the cost of mate guarding by males, thereby increasing the possibility for females to solicit extra-pair paternities to the cuckolding male they have chosen. Finally, we discuss why correlatives approaches do not always show evidence for an effect of breeding density on extra-pair paternity occurrence.

Animals often show a strong antipredatory response when they are exposed to their most deadly predator. In northern vole populations, the least weasel, Mustela nivalis nivalis, is probably the most important predator of voles. Because of its small size and slender body shape, the least weasel is capable of hunting voles in their burrows. However, small voles can potentially escape weasel predation by selecting holes smaller than those weasels can enter. We studied the choice of nest holes and refuges by two species of voles under simulated predation risk. In a laboratory experiment, voles were provided with four nest boxes as refuges, with individually adjusted entrance sizes. When exposed to the odour of a weasel, voles did not choose the smallest opening; they rather seemed to trade full protection for easy and immediate access by choosing the nest box with an intermediate entrance size. When outside the nest at the time when a weasel entered the arena, voles avoided the refuges with the smallest holes. In addition to using refuges on ground level, voles climbed on top of the boxes as an escape reaction, as well as exhibiting a variety of behavioural responses, such as fast running, freezing and sneaking.

1. African Black Oystercatchers Haematopus moquini select nest sites according to macrohabitat type, local substratum, the presence of plants and other objects at and close to the nest, and shelter. Nest sites in both structurally complex and simple habitats are highly cryptic. 2. Eggs are cryptic and there are consistent differences in the patterns of pigmentation between first and second laid eggs within a clutch but between females of the same population overall clutch pattern is similar and only the intensity of pigmentation varies. Within-clutch differences in egg markings did not result in differential predation rates of first and second eggs, but increases nest complexity and hence the crypticity of the clutch. 3. Nests are usually sited adjacent to feeding territories. When such sites are not available, birds “leapfrog” over the territories of neighboring pairs rather than attempt nesting in marginal habitats, and may nest several hundred metres away from their feeding territories. Eggs of “leapfrogging” birds had the highest survival and hatching rates in the study area.

In a species with biparental care two parents cooperate to provide the appropriate amount of care for the young. Recent theoretical treatments consider the evolutionarily stable investment strategy. Under most conditions, the parental investment of the two partners should be negatively correlated, with the shortfall of one partner being partially compensated for by the other. Previous experimental manipulations of biparental care have involved removal of one partner, yet the response of a widowed bird may differ from that of a mated bird whose partner is doing less than its fair share of parental care. We present the first data involving subtle manipulations of sex differences in parental care where both partners continue to care for the young. This study involves pairs in a nestbox colony of european starlings (Sturnus vulgaris L.) with all brood sizes manipulated to five chicks. Pairs were randomly assigned to three groups: (i) male parental care reduced; (ii) female parental care reduce; and (iii) control pairs. Parental care was manipulated by attaching small weights to the base of a bird's tail feathers. Regardless of sex, nest visitation rate was reduced in the weighted birds with an incomplete compensatory increase by their unweighted partner. Additional parental duties were also considered, including shifts in prey type delivered to the nest, in both weighted birds and their partners. The shift in diet and the overall lower total visitation rate in experimental nests contributed to slower chick growth and lower chick weights than in control nests. The data accord with models suggesting that equality of invesment in biparental species is evolutionarily stable, but reveal new dimensions of parental response that need to be taken into account in theoretical treatments.

The orientation of golden hamsters during their return from a food source at the centre of an experimental arena to their nest at the arena's periphery was examined. The experiments took place under different visual conditions and involved conflicting spatial information, which the animal could collect either en route, during the outward journey to the food source, or on site, at the point of departure of the return trip. When the arena and nest box were rotated before the start of the trial (so that the hamsters started the outward journey from different points in absolute space), the animals returned directly to the point of departure of each particular hoarding trip when tested under infra-red light. Deprived of visual cues, they resorted to route-based information, i.e. they depended on the registration and computation of cues which had been generated during the outward journey. If tested under the ordinary room lights, the animals returned in a constant direction towards the usual location of the nest box, but with slight deviations towards the changing point in space where they had initiated each hoarding trip. They therefore relied mainly on stable location-based features from the distant visual background which they had associated with the standard location of the nest entrance; at the same time, however, they were also influenced by information derived from the outward journey. If exposed to a weak light source from outside the arena, the animals' homing behaviour reflected the simultaneous influence of three categories of information: (a) The light as a stable, location-based cue which the subjects had associated with the usual location of the nest; (b) the registration of the previous outward journey by means of internal (idiothetic) path-dependent variables; and (c) the registration of the previous outward journey using the light as visual reference. These results illustrate the interplay and functional hierarchy of various categories of spatial information which the hamster, active at dusk and at night, can use in laboratory conditions as well as in its natural habitat.

The social behavior and reproductive success of individual house mice were monitored in an enclosure. The subjects were obtained from natural populations and were selected on the basis of the genotype at five genetic loci. Male mice formed a three-tiered social hierarchy. In two replicate experiments α-dominant males formed two well defined territories which they constantly patrolled and strongly defended for periods of 120–200 days. The subdominant and subordinate males resided along the periphery of the enclosure and were most active when the dominant males were resting. In the second experiment the subdominant males were able to establish territories after a period of 140 days. Reproductive females generally confined their movements to within the territorial boundaries of a single α-dominant male. Once a female occupied a nest box within a particular male's territory, she rarely entered the territory of the other α-dominant male. Genetic analysis of five segregating loci enabled ascertainment of the paternity of 13 of the litters born and indicated a strong association between social dominance and reproductive success. There was only one instance of gene flow between social breeding units. The genetic analysis therefore indicates that social organization has the potential to restrict gene flow within local populations of house mice.

Aposematic (warning) signals of prey help predators to recognize the defended distasteful or poisonous prey that should be avoided. The evolution of aposematism in the context of predation has been in the center of modern ecology for a long time. But, the possible roles of aposematic signals in other ecological contexts have been largely ignored. Here we address the role of aposematic signals in competition between prey and predators. Bumblebees use visual and auditory aposematic signals to warn predators about their defenses. For 2 years, we observed competition for nestboxes between chemically defended insects, Bombus ardens (and possibly also Bombus ignitus), and cavity nesting birds (Parus minor and Poecile varius). Bumblebees settled in 16 and 9 % of nestboxes (in 2010 and 2011 breeding seasons, respectively) that contained bird nests at the advanced stage of nest building or at the stage of egg laying. Presence of bumblebees prevented the birds from continuing the breeding activities in the nestboxes, while insects took over the birds’ nests (a form of kleptoparasitism). Playback experiments showed that the warning buzz by bumblebees contributed to the success in ousting the birds from their nests. This demonstrates that aposematic signals may be beneficial also in the context of resource competition. Electronic supplementary material The online version of this article (doi:10.1007/s00265-013-1553-2) contains supplementary material, which is available to authorized users.

The initial stage of pair formation of the pied and the collared flycatcher (Ficedula hypoleuca and F. albicollis) was simulated in a field experiment. Male dummies positioned near nest boxes and “singing” by means of tape-recorded song from loudspeakers offered prospecting females a nest box combined with an automatic trap. An unequivocal demonstration of female arrival at the male's territory was obtained by the trapping of the female. Control nest-box traps were provided with silent dummies. Factors other than male song causing female attraction were controlled by a daily switch of the position of “singing” and silent dummies. Nine out of ten females were caught in nest-box traps with “singing” dummies (one-tailed binomial test, P=0.011). The result supports the hypothesis that male song functions in mate attraction. This hypothesis has never previously been tested in field experiments although circumstantial evidence for it has been available.