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In birds and mammals, juvenile and adult foragers are often found apart from each other. In this study, we found this is also true for red knots, Calidris canutus canutus, wintering on the intertidal flats of Banc d'Arguin, Mauritania. Not only did juveniles feed separately from adults, they also fed at places where they were more vulnerable to predation by large falcons. That the dangerous areas used by juveniles were no better feeding areas led us to reject the food-safety trade-off that explained age-related distribution differences in many earlier studies. Instead, juveniles were displaced by adults in dyadic interactions which suggests that they suffered from interference from adults. Juveniles retreated to feeding areas that were more dangerous and yielded lower intake rates, and coped by extending foraging time by using higher, nearshore intertidal areas that were exposed for longer. When disturbed by predators in these nearshore areas, juveniles continued feeding whereas adults left. Thus, rather than compensating for increased predation danger by higher intake rates, on the Banc d'Arguin red knot juveniles foraged for longer.

Subspecies Calidris canutus islandica of the Red Knot breeds on the arctic tundra of northeastern Canada and northern Greenland and winters along the coasts of northwestern Europe. During northward migration, it stops over in either Iceland or northern Norway. It has been assumed that it does the same during southward migration. Using ratios of stable carbon isotopes (δ 13 C) in whole blood, blood cells, and plasma, we investigated evidence for a stopover in Iceland en route from the breeding grounds to the Dutch Wadden Sea. With the expected diet (shellfish) and stopover duration at Iceland (12-15 days, maximum 17 days) and the turnover rates of blood cells (15.1 days) and plasma (6.0 days), Red Knots that stopped in Iceland should arrive with a blood (cell) δ 13 C midway between a tundra (-24.7[per thousand]) and a marine value (-14.0[per thousand]) and a plasma δ13 C approaching the marine value (-15.3[per thousand]). However, many adults arriving at the Wadden Sea had δ13 C ratios in blood (cells) and plasma below these levels, and some arrived with clear tundra signals in blood cells, suggesting that they skipped Iceland during southward migration. Surprisingly, available data suggest this also to be true for juveniles during their first southward migration. The δ 13 C signature of second-year birds confirmed that they oversummered in the Wadden Sea. Our findings contradict the largely untested idea that juvenile shorebirds make more stopovers than adults as well as the idea that the migration between the Nearctic and Europe is necessarily a two-leg process.

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.

We here describe howthe densities of Baltic tellins Macoma balthica decreased with two orders of magnitude over the last 20 years. Densities of several 100 Macoma per square meter are now very rare in this once very widespread species. Local declines have followed severe forms of habitat disturbance, notably mechanical dredging for seafood, especially cockles Cerastoderma edule.

In this study we aim to provide a basic description and comparison of the spatial distribution and population structure of the common intertidal mollusc species, sampled within a single winter along a latitudinal gradient of different soft-sediment areas spanning 8° of latitude (46°–54°N) and 900 km of distance in northwest Europe. Sediment samples were collected from December 2003 to early March 2004 in the Wadden Sea (The Netherlands), the Wash (United Kingdom), Mont Saint–Michel Bay (France) and two bays on the central French Atlantic coast in south of Brittany. Core-sampling over 250 m grids allowed us to cover 3–30 km² at nine separate intertidal subsites, with a grand total of 2,103 points visited. Among the 15 bivalve and 8 gastropod species collected, we focused on the four most common and abundant bivalve species (Cerastoderma edule, Macoma balthica, Scrobicularia plana and Abra tenuis) that together represented 96% of all collected bivalves, and on the mudsnail Hydrobia ulvae that comprised 99% of all collected gastropods. C. edule and M. balthica were the most widespread bivalves, with higher densities occurring at higher latitudes. S. plana and A. tenuis were more abundant at southern sites, both with a clear preference for muddy sediments. The mudsnail H. ulvae occurred commonly and in comparable densities at all study sites, except in Mont Saint–Michel Bay where it was very rare. Mean sizes of the common molluscs were highly variable between sites, without clear north–south gradients. The mollusc distribution patterns at the five intertidal areas and nine subsites were predominantly site-specific. Mollusc community composition showed greater similarity within than between the regions north and south of the Brittany peninsula.

We have used the isotope dilution technique to study changes in the body composition of a migratory shorebird species (Red Knot, Calidris canutus) through an assessment of the amount of body water in it. Birds were quantitatively injected with a dose of water with elevated concentrations of H-2, O-17, and O-18. Thereafter, blood samples were taken and distilled. The resulting water samples were analysed using an isotope ratio mass spectrometry (for H-2 and O-18 only) and a stable isotope ratio infrared laser spectrometry (H-2, O-17, and O-18) to yield estimates of the amount of body water in the birds, which in turn could be correlated to the amount of body fat. Here, we validate laser spectrometry against mass spectrometry and show that all three isotopes may be used for body water determinations. This opens the way to the extension of the doubly labelled water method, used for the determination of energy expenditure, to a triply labelled water method, incorporating an evaporative water loss correction on a subject-by-subject basis or, alternatively, the reduction of the analytical errors by statistically combining the O-17 and O-18 measurements.