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In order to determine the effects of seed burial and flooding depths on the emergence and seedling growth of watergrass (Echinochloa oryzoides) and barnyardgrass (E. crus-galli), this experiment was conducted in a covered free air condition, at the Rice Research Institute of Iran, Rasht in summer of 2009. The treatments included four seeding depths (0.1 (seeds were mixed with the top soil), 2, 4 and 6 cm), and four flooding depths (0.1 (saturated soil with no standing water), 3, 6, and 9 cm) arranged as a factorial arrangement in a randomized complete block design with three replications. After 28 days, number of emergent seedlings, weed seedlings height, root and shoot fresh weight, leaf and root area were measured, and emergence percentage and root/shoot ratio were calculated. Since no germination of seeds was observed at the 6 cm sowing depth, this treatment wasn’t presented in results. Measured or calculated variable per unit was significantly different between two species and between various levels of seed burial and flooding depths. These variables also influenced by the interactions of the mentioned factors with an exception of emergence percentage. Emergence percentage and growth characteristics except for plant height were higher in E. oryzoides than E. crus-galli when weed seeds were located in the depth of 0.1 centimeter. In seeds located in the depth of 2 centimeters, all variables were higher in E. oryzoides than E. crus-galli. In both species, the higher values for mentioned variables was observed in the saturated condition of soil, when weed seeds were located at the depth of 0.1 centimeter and reduced with increasing flooding and seed burial depth. Results indicated that flooding condition with adequate height caused major limiting effects on emergence, establishment and growth of two Echinochloa species, especially when the seeds were deeply buried; but at decreased height specially saturated soils, emergence and growth of E. oryzoides were significantly more than E. crus-galli.

Besides the “normal” challenge of obtaining adequate intake rates in a patchy and dangerous world, shorebirds foraging in intertidal habitats face additional environmental hurdles. The tide forces them to commute between a roosting site and feeding grounds, twice a day. Moreover, because intertidal food patches are not all available at the same time, shorebirds should follow itineraries along the best patches available at a given time. Finally, shorebirds need additional energy stores in order to survive unpredictable periods of bad weather, during which food patches are covered by extreme tides. In order to model such tide-specific decisions, we applied stochastic dynamic programming in a spatially explicit context. Two assumptions were varied, leading to four models. First, birds had either perfect (ideal) or no (non-ideal) information about the intake rate at each site. Second, traveling between sites was either for free or incurred time and energy costs (non-free). Predictions were generated for three aspects of foraging: area use, foraging routines, and energy stores. In general, non-ideal foragers should feed most intensely and should maintain low energy stores. If traveling for such birds is free, they should feed at a random site; otherwise, they should feed close to their roost. Ideal foragers should concentrate their feeding around low tide (especially when free) and should maintain larger energy stores (especially when non-free). If traveling for such birds is free, they should feed at the site offering the highest intake rate; otherwise, they should trade off travel costs and intake rate. Models were parameterized for Red Knots (Calidris canutus) living in the Dutch Wadden Sea in late summer, an area for which detailed, spatially explicit data on prey densities and tidal heights are available. Observations of radio-marked knots (area use) and unmarked knots (foraging routines, energy stores) showed the closest match with the ideal/non-free model. We conclude that knots make state-dependent decisions by trading off starvation against foraging-associated risks, including predation. Presumably, knots share public information about resource quality that enables them to behave in a more or less ideal manner. We suggest that our modeling approach may be applicable in other systems where resources fluctuate in space and time. [KEYWORDS: Calidris canutus ; daily routine ; foraging behavior; patch use ; Red Knot ; state-dependent decisions ; stochastic dynamic programming ; tidal regime ; Wadden Sea.]

International audience; Seagrass patches interspersed in a sediment matrix may vary environmental conditions and affect feeding habits of consumers and food-web structure. This paper investigates diet shifts between bare sediments and a Zostera noltei (Hornemann, 1832) meadow for three facultative deposit-feeding macrofaunal consumers, notably the bivalve Scrobicularia piano (da Costa, 1778), the polychaete Hediste diversicolor (O.T. Muller, 1776), and the gastropod Hydrobia ulvae (Pennant, 1778). In July 2008, one eelgrass meadow and two bare sediment locations were chosen in the Mondego estuary (40 degrees 08" N, 8 degrees 50'W, Portugal) and sampled for stable isotope signatures (delta C-13 and delta N-15) of macrofauna consumers and some of their potential basal food sources, such as sedimentary organic matter (SOM), microphytobenthos (MPB), seagrass shoots, leaves and seaweeds laying on the surface sediment. The delta N-15 of H. diversicolor was 3% higher in the eelgrass meadow than in bare sediment, indicating a change of trophic position, whereas the Bayesian stable-isotope mixing model showed that S. piano assimilated more macroalgal detritus than microphytobenthos in the eelgrass bed. Such habitat-related diet shifts have the potential to change structure and spatial dynamics of benthic food webs.

Understanding trawling impacts on the benthic ecosystem depends to a large extent on the ability to estimate trawling activity at the appropriate scale. Several studies have assessed trawling at fine spatial scales yearly, largely ignoring temporal patterns. In this study, we analysed these temporal patterns in beam trawl effort intensity at 90 stations of the Dutch continental shelf of the North Sea for a period of 10 years, at a fine temporal (weekly) and spatial (110 × 70 m) scale using Vessel Monitoring by Satellite (VMS) data. Our results show that trawling is aggregated in time and shows clear seasonality, related to the behavior of the fleet and migration patterns of the target fish species. The temporal patterns affect the overall impact on and the recovery of the benthic community, as is illustrated with a benthic population model. Our results imply that trawling impact studies using high-resolution data like VMS should take account of the possibility of temporal aggregation and seasonality in trawling to improve the assessment of the impact of trawling on the population dynamics of benthos.

Population dynamics of the tellinid clam Macoma balthica (L.) were studied at a highly dynamic intertidal sandflat in the Westerschelde estuary, south-western Netherlands. In order to study temporal development of density and population structure (12 size classes from 0.3 to 20 mm), 3 point sampling stations were sampled fortnightly from March 1992 to March 1998. Within the same period, spatial population dynamics was studied seasonally on a spatial grid (700 x 800 m, 43 plots, distance between the plots 100 m), which covered a range in bedlevel height from-50 to +140 cm relative to mean tide level. Quantitative estimations of early recruitment, growth, and survival plus migration were calculated from the temporal and spatial population data. Early recruitment was highest at the higher tidal levels, where the sediment contained the smallest sand grains. In that same area, the highest disappearance of the juvenile M, balthica, caused by emigration and/or mortality, was observed. In the lower intertidal area, where the sediment contained larger sand grains, the number of recruits in the successive classes increased after Size Class 2 to 3 mm. Based on further analysis of the population data, this increase in the number of recruits is concluded to have been mainly caused by immigration of M. balthica to the lower tidal levels. Since there seemed to be no important immigration into the entire spatial grid population, the migration may have been directed from the higher intertidal levels towards the lower. The strong initial early-recruitment pattern, supposed to be comprised of passive processes, became weaker for the successive size classes after Size Class 2 to 3 mm, which resulted in a spatial distribution of Size Class 7 to 8 mm that was neither related to bedlevel height nor to sand grain size. Therefore, active migration processes are suggested to have been of higher importance than passive migration processes. [KEYWORDS: passive resuspension, active migration, secondary settlement, passive deposition, habitat selectivity]

During deposit feeding on benthic microalgae, the siphon of buried tellinid bivalves like Macoma balthica is vulnerable to nipping by plaice Pleuronectes platessa and other flatfish. We experimentally tested the hypothesis that siphon nippers facilitate predation on the entire bivalve (by shorebirds, for example) by inducing a decrease in burying depth. Three experiments in May and in September, during which likely siphon nippers (juvenile plaice P. platessa) were allowed to feed on M. balthica siphons, demonstrated that the bivalves indeed lost siphon mass and came closer to the surface. However, the strength of the burying response and the speed of recovery of siphons after nipping varied greatly between experiments, partly as a consequence of body condition-related differences in initial burying depth. Our experimental study confirms that siphon nippers can enhance the availability of bivalve prey to probing predators, and will thus facilitate avian predation on intertidal flats.

We investigated the fledging probability of oystercatcher, Haematopus ostralegus, chicks as a function of hatching order, brood size, territory quality and food availability. Sibling dominance was related to the hatching order in both low- ('leapfrogs') and high-quality ('residents') territories. Differences in hatchling mass might have aided the establishment of a dominance hierarchy, since breeders produced small late eggs and hatchlings. These mass differences were most pronounced in leapfrogs, and in large broods in years with lower food availability ('poor' years). Late hatchlings fledged less often and with lower body masses compared to early hatchlings in all situations. Leapfrogs produced smaller broods and hatched their broods more asynchronously in poor years than leapfrogs breeding in years with more available food ('good' years) and residents breeding in both poor and good years. Large brood sizes resulted in lower survival of hatchlings in poor years. These results favour the 'brood reduction' hypothesis. However, contrary to the expectations of this hypothesis, hatching order also affected fledging success in residents. Moreover, large brood size resulted in higher survival of hatchlings in good years, particularly in residents. Thus, although large broods experienced losses due to sibling competition in some years, they nevertheless consistently produced more fledglings per brood in all years, both as leapfrogs and residents. We believe this effect is due to parental quality correlating with initial brood size. Most leapfrogs, at best, fledged one chick successfully each year, losing chicks due to starvation. Nevertheless, leapfrog broods were reduced in size after hatching significantly less quickly than resident broods. These results suggest that breeders lay and hatch insurance eggs to compensate for unpredictable losses due to the high predation rates on both nests (ca 50%) and chicks (ca 90%), in accordance with the 'nest failure' hypothesis.

1. The timing of prey exploitation by oystercatcher Haematopus ostralegus L, was predicted from detailed knowledge of the characteristics of its prey, the clam Scrobicularia plana da Costa. 2, Growth, mortality and depth distribution of a single cohort of Scrobicularia were monitored on a tidal flat in the Dutch Wadden Sea during a period of 7 years. 3. A random search model predicted for every sampling date the encounter rate with buried clams providing oystercatchers vary the depth at which they probe their bill into the substrate. The time needed to handle encountered prey was estimated from experimentally determined relationships of handling time with clam size and handling time with burying depth. Encounter rate, handling time and body weight of the potential prey items were used in an optimal prey choice model to predict the average intake rate of an oystercatcher as a function of probing depth. 4, The predicted intake rate reached a profitable level only during three restricted periods. During the first period, a relatively high intake rate could only be achieved by probing extremely deeply, and the mortality rate of the clams remained at a low level. During the other two periods, maximal intake rate was reached by probing only the upper few cm of the substrate, and this time there was a high mortality in the prey population. 5. Although the observed peaks in clam mortality coincided with the predicted exploitation periods, they could not be explained fully by the estimated predation pressure. 6. Prey switching was predicted and confirmed during several exploitation periods. 7. Oystercatcher densities during winter peaks in bird abundance showed a tight relationship with predicted maximum intake rates.

We predict the intake rate and prey choice of Oystercatchers feeding along the Frisian coast, Dutch Wadden Sea, combining the optimal prey choice model (Charnov 1976) with detailed measurements of the widely fluctuating food supply. Assuming that the birds maximize their intake rate, the birds should never eat Mussels Mytilus edulis during 10 years of observa- tions, Mya arenaria during two short periods, Macoma blathica and Scrobicularia plana during most summers and Cockles Cerastoderma edule in most winters. Observations on feeding Oystercatchers confirmed the predictions. Due to the seasonal variation in burying depth of Scrobicularia and Macoma, these prey were in winter, if not inaccessible, hardly worthwhile exploiting because of the increase of handing time and searching time with burying depth. Hence, the seasonal variation in intake rate was very large in these deep-living prey compared to surface prey, such as Cockles and Mussels. Consequently, Oystercatchers usually switch from surface to deep-living prey in spring and back to surface prey in autumn in order to maximize their intake rate. Oystercatchers will never achieve a high intake rate when they feed on small prey, even when these prey would occur in extremely high densities. The reason for this is that the yield of small prey during handling is even less than the intake rate during feeding of 1 mg ash-free dry weight (AFDW) s(-1) which Oystercatchers need to meet their energy demands juring the limited feeding periods in the tidal habitat. Since Oystercatchers eat only large bivalves, they might be vulnerable because cohorts of prey may disappear completely before they can be harvested. Despite the very large annual variation in the biomass of the different prey species in the Wadden Sea, the total food supply harvestable by Oystercatchers is large enough for them to stay ir, the area, unless ice covers the tidal flats. However, Oystercatchers cannot survive in the Wadden Sea when their diet is restricted to one or two prey species. They need to switch between at least 3 or 4 prey species. For the same reason, the birds have to roam over feeding areas measuring ar least some ten's of km(2). The winter remains a difficult period, however. The mortality is higher in winter than in summer and increases with the severity of the winter. Besides, the winter mortality increases when the food consumption is reduced, due to either a low intake rate and/or a short feeding time. Therefore, the wintering numbers of Oystercatchers in the Wadden Sea are limited during circumstances which occur in only some of the winters, viz. when ice covers the feeding areas and the harvestable food supplies are low.The total biomass of the five bivalve species in the study area amounted to 81 g ash-free dry flesh (AFDW) m(-2), on average. The annual production was 56 g m(-2), but only 32 g m(-2) can be considered as exploitable by Oyster catchers. Oystercatchers did not han est the 9 g m(-2) year(-1) produced by large Mya living out of I-each of the bill, nor the 5 g m(-2) produced by bivalves too small to be eaten by Oystercatchers. Moreover 9 g m(-2) disappeared during disasters (e.g. frost) and could not be eaten by birds. Oystercatchers consumed 12 g m(-2) year(-1), on average, thus more than the 10 g m(-2) taken by all other shorebird species together. Half of the prey biomass disappearing due to mortality between August and March could be attributed to Oystercatcher predation. The predation pressure by Oystercatchers was much lower iu Scrobicularia and Macoma. In contrast, 80% of the second year Mya was eaten by Oystercatchers in some months. The numbers of Oystercatchers feeding in the study area were weakly related to the annual variations in the total food supply, but strongly related to those of the harvestable food supply. This high correlation must be due to two causal relationships: the bird density increases with the intake rate, and intake rate increases with the harvestable food supply.

In this paper an analysis is made of subtle behavioural differences between adult male and female Ovstercatchers feeding on Macoma balthica under field conditions and in captivity. Macoma is a tellinid bivalve that in the Dutch Wadden Sea is mainly preyed upon during spring and summer when it is buried at a shallow depth. males lift Macoma more, whereas females handle them mostly in situ. Both sexes handle a Macoma in situ faster than one lifted. Time loss of males in handling more lifted Macoma is compensated by the larger size of lifted Macoma, which yields more flesh. The time the birds need to find an edible Macoma is similar for both sexes, resulting in equal mean food intake rates for males and females in the field. Lifted Macoma are generally hammered and, since males with their short strong bills are more likely to hammer bivalves than females, this difference in bill morphology might explain why males more often lift Macoma than do females, especially as hammering produces a blunt bill tip which would reduce efficiency at opening Macoma in situ. However, none of the selected bill morphology variables showed a relationship within the sexes that explained the differences between the sexes.

Prey species have different morphological and behavioural adaptations to escape their predators. In this paper we review how these prey defenses affect prey profitability and intake rate for one predator, the Oystercatcher. Four rules govern profitability. First, within each species large prey are more profitable than small prey, because flesh content increases more steeply with prey size than handling time. Second, soft-bodied prey, such as worms and leatherjackets, which can be swallowed whole, are much more profitable than armoured prey, such as bivalves, which Oystercatchers have to open before the flesh can be extracted from the shell. Third, heavily armoured surface-dwelling prey, like Mussels and Cockles, are the least profitable prey of all, even if the armour is bypassed through stabbing the bill between the valves. Fourth, within the burying prey species, the profitability of prey decreases with depth. Hence burying bivalve species that bury in winter at larger depth than in summer, are in winter, if not our of reach of the bill, anyway less profitable. Despite the large differences between the profitabilities of the various prey species, the intake rates do not differ much when the prey species are com pared, presumably because prey with a low profitability are only exploited if the search time is relatively short, i.e. if the density of harvestable prey is high. On the other hand, within each species, the intake rate goes up if larger, more profitable prey are taken. Thus, if the birds have to feed on smaller prey specimens, they fail to fully compensate for the low profitability by an increase in the rate at which these prey are found. Although the profitability of prey differs seasonally due to the variation in the prey condition, only a small seasonal variation in the intake rate was found. Because burying bivalves and soft-bodied worms bury deeper and are less active in winter, Oystercatchers necessarily rely on bivalves living at, or just beneath, the surface at that time of year.

Oystercatchers Haematopus ostralegus breeding on the isle of Schiermonnikoog in the Dutch Wadden Sea: switched from a diet dominated by the bivalve Macoma balthica in late spring to a diet dominated by the annelid worm Nereis diversicolor in early summer. Although all Oystercatchers switched, the timing and the magnitude of the switch differed between individuals. Since searching for Macoma appeared incompatible with searching for Nereis, we expected individuals to search for the prey species yielding the highest intake rate for a given period in the season. Some analyses clearly supported this suggestion, while the results of others were at best ambiguous. Although the density of large Macon!a did not change, the intake rate of Macoma declined during the summer, due to a decline in the capture rate of Macoma. This may have resulted from an increase in the burying depth and a decline in the condition of Macoma, as this forced the Oystercatchers to prey on an increasingly smaller fraction of the Macoma population. Intake rate on Nereis was not related to the density of Nereis, which tripled during the course of the study in 1986, while intake rate seemed to decline in late summer after having reached a peak in early summer. This peak was possibly due to a high surface activity of Nereis. Thus, the diet switch may have been due to an increase in the burying depth and a decline in the condition of. Macoma, or to an increase in the surface activity of Nereis, or both. Problems of interpretation arose primarily from the consistent differences between individuals in prey choice and the lack of independent measures of prey availability.

Three individually-tested adult Oystercatchers took different proportions of shallow-buried bivalves Mya arenaria and Scrobicularia plana from a mixture on offer in an experimental situation. Two birds, taking mainly or exclusively Scrobicularia, selected the species as predicted by a random search model based on encounter rate. The third bird actively selected for the less abundant Mya. All birds showed size selection within the prey species. Diet composition was correlated with the efficiency of the birds in handling the prey items. Optimal foraging theory adequately explained the observed prey species- and size selection by the birds. A short-term learning effect was found in two of the Oystercatchers. These birds managed to reduce their handling time during the course of the experiments, mainly by cutting the flesh loose from the shell faster. Cutting was always the most time-consuming component of prey handling but relatively more when a bird took the less preferred prey species. By rapidly improving their cutting technique for a specific prey, the birds increased their profitability. This adaptive behaviour contradicts the view of the Oystercatcher as a conservative specialist, which has problems when alternative prey should be taken in times of food shortage, although under natural conditions prey switching is hampered by morphological constraints.

This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding benthic bivalves under natural conditions. Were this to be so, it would imply that foraging and predator avoidance place conflicting demands on these animals, since an increase in the feeding radius on the surface would be associated with a decrease in the burying depth, and vice versa. The paper presents methods with which to measure siphon length and equations to transform siphon weight to siphon length in two benthic bivalves, Scrobicularia plana and Macoma balthica. Relatively heavy siphons are longer, but also thicker, than lightweight ones. We conclude that most individual bivalves stretch their siphons fully while feeding. However, bivalves with heavy siphons keep part of the siphon in reserve within the shell. Siphon cropping therefore results in an immediate reduction in siphon length, unless the siphon is heavy and a reserve is available. The feeding radius of M. balthica and S. plana is a linear function of shell size. S. plana use half of their siphon length to feed on the surface and the other half to bury themselves, but if the siphon weight is below average, the proportion extended over the surface decreases with siphon size. However, among the animals with a short siphon, those with a good body condition take no risks and live as deeply as possible. A comparison between species reveals that the weight of the extended inhalant siphon per cm amounts, on average, to 0.6% of the total body weight.

In August–October 1988–1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10 000 to 25 000 birds. Over 100 000 knots were recorded on three occassions. Knots feed in large flocks, individual birds usually experiencing 4 000 to 15 000 flock-mates. The Siberian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964–1992 there were no clear trends in the number of knots, but canutus -knots were particularly abundant in July–August 1991, whereas in 1992 both subspecies were absent. Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distributon of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year and season, i.e. Macoma (main prey when their harvestable biomass densities were greater than ca 0.8 g AFDM per m 2 ) and Cerastoderma , and by the avoidance of situations where they run the risk of attack by bird-eating birds. Flocks of knots covered most of the intertidal flats in the Western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km 2 , much larger than the equivalent area used by knots on their wintering grounds in Mauritania (10–15 km 2 ), a difference that is correlated with prey spectrum, prey availability and predictability.

Deposit-feeders are the dominant bioturbators of aquatic sediments, where they profoundly impact biogeochemical processes, but they are also vulnerable to both lethal and non-lethal predation by a large variety of predators. In this study, we performed a series of experiments to test the effects of predation avoidance on the feeding activity and sediment reworking intensity of the deposit-feeding bivalve Macoma balthica. Feeding activity at the sediment–water interface and sediment reworking intensity (vertical displacements of inert particle tracers) were monitored using image analysis techniques for treatments including and excluding the predatory shrimp, Crangon crangon. Detection of C. crangon by M. balthica resulted in an immediate retraction of the feeding siphon and a reduction in feeding activity. M. balthica also buried deeper into the sediment in the presence of C. crangon. This predator avoidance behaviour indirectly affected sediment reworking modes and rates, increasing the thickness of the bioturbated sediment layer as well as the non-local transport of sediment particles at depth. Conversely, feeding activity and sediment reworking processes remained unaffected when C. crangon was present, but isolated from the sediment, suggesting that predator perception in M. balthica is tactile (i.e. induced by direct encounter) rather than being chemosensory. Collectively, these results demonstrate that predatory avoidance behaviour by benthic infauna can significantly impact benthic bioturbation and the incorporation of organic matter into the benthic food web.

We quantified the prey selection and intake rate of a wading bird, knot Calidris canutus, when 6 different, intertidal prey species, the mud snail Peringia ulvae and the bivalves Macoma balthica, Cerastoderma edule, Mya arenaria, Scrobicularia plana and Mytilus edulis, were abundant. Knot usually search for food by randomly probing into the mud. Prey selection may be described by 5 rules: (1) Prey are not ingestible by knot if the circumference is more than 30 mm. (2) Prey are not accessible to knot when they lie buried deeper than 2 to 3 cm. (3) The probability of a prey organism being detected depends on its surface area, measured in the horizontal plane. (4) Prey are ignored when they are unprofitable, i.e. when the rate of intake while handling the prey is below the current overall average intake rate during feeding. (5) Knot prefer thin-shelled to thick-shelled prey species, possibly because a high inorganic content has an inhibitory effect on the rate at which energy can be extracted from the food. The first rule of ingestion is set by the gape width, and is therefore invariable. The fraction of the prey which is accessible varies according to the probing depth of the bird. The lower size threshold of prey taken is determined by the profitability rule and so varies according to the current feeding rate of knot. Unfortunately for knot, the majority of the preferred thin-shelled prey live out of reach of the bill, whereas the thick-shelled species live at the surface. Medium-sized Macoma balthica is the best compromise available in the 6-species mix.

Optimality reasoning from behavioural ecology can be used as a tool to infer how animals perceive their environment. Using optimality principles in a ‘reversed manner’ may enable ecologists to predict changes in population size before such changes actually happen. Here we show that a behavioural anti-predation trait (burrowing depth) of the marine bivalve Macoma balthica can be used as an indicator of the change in population size over the year to come. The per capita population growth rate between years t and t +1 correlated strongly with the proportion of individuals living in the dangerous top 4 cm layer of the sediment in year t : the more individuals in the top layer, the steeper the population decline. This is consistent with the prediction based on optimal foraging theory that animals with poor prospects should accept greater risks of predation. This study is among the first to document fitness forecasting in animals.

Many species of waders forage extensively at night, but there is very little information on the relevance of this behaviour for the energy budget of waders wintering in estuarine wetlands. Quantitative data on diurnal and nocturnal intake rates can indicate the extent to which birds need to forage at night to supplement their diurnal energetic intake, or rather show a preference for nocturnal foraging.We compared day and night foraging behaviour, diet, and energy consumption of several wader species in the Tejo estuary, Portugal. There were significant differences between diurnal and nocturnal foraging behaviour. In general, birds moved less at night and scolopacid waders tended to use more tactile foraging methods. Although birds consumed the same type of prey in the two periods, the relative importance of each type changed. Overall, energy consumption was higher during the day except in grey plover, which achieved higher crude intake rates at night.Our results support the assertion that night foraging is an important part of the energy balance of waders during late winter, but that in most species it is less profitable than diurnal foraging. (c) 2008 Elsevier Masson SAS. All rights reserved.

Eutrophication due to high anthropogenic nutrient loading has greatly impacted ecological processes in marine coastal waters and, therefore, much effort has been put into reducing nitrogen and phosphorus discharges into European and North-American waters. Nutrient enrichment usually resulted in increase of biomass and production of phytoplankton and microphytobenthos, often coinciding with shifts in species composition within the primary producer community. Consequences of increasing eutrophication for higher trophic levels are still being disputed, and even less is known about the consequences of nutrient reduction on coastal food webs. Here, we present 30-year concurrent field observations on phytoplankton, macrozoobenthos and estuarine birds in the Dutch Wadden Sea, which has been subject to decades of nutrient enrichment and subsequent nutrient reduction. We demonstrate that long-term variations in limiting nutrients (phosphate and silicon) were weakly correlated with biomass and more strongly with community structures of phytoplankton, macrozoobenthos and estuarine birds. Although we cannot conclusively determine if, and if so to what extent, nutrient enrichment and subsequent nutrient reduction actually contributed to the concurrent trends in these communities, it appears likely that part of the variance in the studied coastal communities is related to changes in nutrient loads. Our results imply that nutrient reduction measures should not ignore the potential consequences for policies aimed at bird conservation and exploitation of marine living resources.

1. The timing of prey exploitation by oystercatcher Haematopus ostralegus L, was predicted from detailed knowledge of the characteristics of its prey, the clam Scrobicularia plana da Costa. 2, Growth, mortality and depth distribution of a single cohort of Scrobicularia were monitored on a tidal flat in the Dutch Wadden Sea during a period of 7 years. 3. A random search model predicted for every sampling date the encounter rate with buried clams providing oystercatchers vary the depth at which they probe their bill into the substrate. The time needed to handle encountered prey was estimated from experimentally determined relationships of handling time with clam size and handling time with burying depth. Encounter rate, handling time and body weight of the potential prey items were used in an optimal prey choice model to predict the average intake rate of an oystercatcher as a function of probing depth. 4, The predicted intake rate reached a profitable level only during three restricted periods. During the first period, a relatively high intake rate could only be achieved by probing extremely deeply, and the mortality rate of the clams remained at a low level. During the other two periods, maximal intake rate was reached by probing only the upper few cm of the substrate, and this time there was a high mortality in the prey population. 5. Although the observed peaks in clam mortality coincided with the predicted exploitation periods, they could not be explained fully by the estimated predation pressure. 6. Prey switching was predicted and confirmed during several exploitation periods. 7. Oystercatcher densities during winter peaks in bird abundance showed a tight relationship with predicted maximum intake rates.

Three individually-tested adult Oystercatchers took different proportions of shallow-buried bivalves Mya arenaria and Scrobicularia plana from a mixture on offer in an experimental situation. Two birds, taking mainly or exclusively Scrobicularia, selected the species as predicted by a random search model based on encounter rate. The third bird actively selected for the less abundant Mya. All birds showed size selection within the prey species. Diet composition was correlated with the efficiency of the birds in handling the prey items. Optimal foraging theory adequately explained the observed prey species- and size selection by the birds. A short-term learning effect was found in two of the Oystercatchers. These birds managed to reduce their handling time during the course of the experiments, mainly by cutting the flesh loose from the shell faster. Cutting was always the most time-consuming component of prey handling but relatively more when a bird took the less preferred prey species. By rapidly improving their cutting technique for a specific prey, the birds increased their profitability. This adaptive behaviour contradicts the view of the Oystercatcher as a conservative specialist, which has problems when alternative prey should be taken in times of food shortage, although under natural conditions prey switching is hampered by morphological constraints.

Oystercatchers Haematopus ostralegus breeding on the isle of Schiermonnikoog in the Dutch Wadden Sea: switched from a diet dominated by the bivalve Macoma balthica in late spring to a diet dominated by the annelid worm Nereis diversicolor in early summer. Although all Oystercatchers switched, the timing and the magnitude of the switch differed between individuals. Since searching for Macoma appeared incompatible with searching for Nereis, we expected individuals to search for the prey species yielding the highest intake rate for a given period in the season. Some analyses clearly supported this suggestion, while the results of others were at best ambiguous. Although the density of large Macon!a did not change, the intake rate of Macoma declined during the summer, due to a decline in the capture rate of Macoma. This may have resulted from an increase in the burying depth and a decline in the condition of Macoma, as this forced the Oystercatchers to prey on an increasingly smaller fraction of the Macoma population. Intake rate on Nereis was not related to the density of Nereis, which tripled during the course of the study in 1986, while intake rate seemed to decline in late summer after having reached a peak in early summer. This peak was possibly due to a high surface activity of Nereis. Thus, the diet switch may have been due to an increase in the burying depth and a decline in the condition of. Macoma, or to an increase in the surface activity of Nereis, or both. Problems of interpretation arose primarily from the consistent differences between individuals in prey choice and the lack of independent measures of prey availability.

The influence of siphon browsing on the feeding behaviour and growth of Macoma bclthica, a deposit-feeding bivalve, was studied in three manipulative experiments. Browsing was simulated by removing part of the inhalant siphon with scissors, or studied by exposing the bivalves to shrimps (Crangon crangon). Smaller siphons resulted in a reduction of the sediment surface area that could be grazed, and a decrease in siphon activity in M. balthica. Disturbance of the siphons by shrimps also reduced siphon activity. Siphons were reduced in size in the presence of shrimps. Immunological analyses of the stomachs of C. crangon identified this species as a consumer of siphon tips. Growth of M. balthica was negatively influenced by siphon browsing, probably due to three interactive factors: extra energy needed for siphon regeneration, less time available for feeding, and a smaller area from which to collect food. Burying depth was reduced in M. balthica with smaller siphons. However, individuals with similar siphon size were buried less deep in the sediment when not exposed to siphon browsing, suggesting an immediate response to absence of browsers.

The relationship between relative body condition (deviation from expected mean body weight) and burying depth was investigated in five macro-zoobenthic species living in a marine intertidal habitat. Body weight increased with depth when animals of the same size were compared. The increase amounted to 50% in the clam Scrobicularia plana, ca. 40% in the worm Nereis diversi-color, 25% in the clam Macoma balthica and 20% in the cockle Cerastoderma edule and the clam Mya arenaria. Only a part of the prey was within reach of some feeding wader species. Therefore prey value may be overestimated if one does not take into account the fact that shallow and accessible prey often have a relatively poor body condition.

In this paper an analysis is made of subtle behavioural differences between adult male and female Ovstercatchers feeding on Macoma balthica under field conditions and in captivity. Macoma is a tellinid bivalve that in the Dutch Wadden Sea is mainly preyed upon during spring and summer when it is buried at a shallow depth. males lift Macoma more, whereas females handle them mostly in situ. Both sexes handle a Macoma in situ faster than one lifted. Time loss of males in handling more lifted Macoma is compensated by the larger size of lifted Macoma, which yields more flesh. The time the birds need to find an edible Macoma is similar for both sexes, resulting in equal mean food intake rates for males and females in the field. Lifted Macoma are generally hammered and, since males with their short strong bills are more likely to hammer bivalves than females, this difference in bill morphology might explain why males more often lift Macoma than do females, especially as hammering produces a blunt bill tip which would reduce efficiency at opening Macoma in situ. However, none of the selected bill morphology variables showed a relationship within the sexes that explained the differences between the sexes.