Article

Age-related distribution of dunlin in the Dutch Wadden Sea.

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Abstract

Analyses the distribution of Calidris alpina from variations in the numbers of birds and their age ratios as determined in 6 localities. Migration routes are also analysed. Results are discussed in relation to the theory of habitat distribution of Fretwell & Lucas (1970).-from Authors

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... The fact that proportional abundance was greatest for the same subsite at both tidal stages for the wader species (Redshank and Grey Plover) suggests that some of the birds may have been foraging preferentially close to their roost sites. This has been shown in previous studies of Oystercatcher (Swennen 1984) and Dunlin Calidris alpina (Have et al. 1984, Dias et al. 2006a where higher status birds tended to roost closer to the best feeding areas. This strategy minimises the distance between roosting and feeding areas and reduces the energy costs of flying between sites (Rogers 2003). ...
... data). Similar observations were reported by Van der Have et al. (1984) for Dunlin in the Wadden Sea of the Netherlands, in autumn, where the juveniles staged in smaller groups close to the mainland shores. Age-related mortalities in Dunlin in a similar context have been reported by Kus et al. (1984). ...
... For example, van den Hout et al. (2014) showed that interference competition from adults forced juvenile red knots (Calidris canutus) to poorer and more dangerous feeding areas. We are as yet unable to assess whether this applies to Pacific dunlins, though van der Have et al. (1984) showed that among dunlins in the Dutch Wadden Sea, juveniles were over-represented in areas with low dunlin density, suggesting that they were somehow excluded from better feeding areas. An alternative hypothesis is that some other site attribute is negatively correlated with foraging rate, so that sites better in foraging terms are poorer in some other respect. ...
Article
Many studies have investigated how foraging behavior such as prey choice varies with factors such as prey size or density. Models of such relationships can be applied “in reverse” to translate easily observed foraging behaviors into assays of habitat attributes that cannot (easily) be measured directly. One such model analyzes the speed of a forager flying between patches, where it captures prey. Faster flight shortens the travel time and hence elevates the intake rate, but is increasingly expensive. The model shows that the net intake rate is maximized at the point at which the energetic cost of flight is equivalent to the net rate of intake. Easy-to-measure flight speeds can thus be translated into hard-to-measure foraging intake rates using established flight power relationships. We studied nonbreeding Pacific dunlins (Calidris alpina pacifica) at 4 intertidal sites on the Fraser River estuary, British Columbia, Canada. These sites differed sufficiently that we expected food availability and hence the attainable foraging rate to differ. We measured interpatch flight speeds of dunlins foraging along the tideline within each site. The measured ground speed, calculated airspeed, and the statistically derived zero-wind effect airspeed all differed significantly between sites, matching in rank order our expectation of habitat quality based on their physical differences. Intake rate estimates ranged from 4.10W (best mudflat) to 3.48W (poorest). We think it unlikely that we would have been able to find such small differences using direct measures of foraging intake.
... De valken mijden ook hier de open wadvlaktes om zoveel mogelijk hun prooien vanuit een hinderlaag te kunnen overrompelen. Juveniele steltlopers worden vaak aangetroffen op plaatsen waar de meerderheid van hun soortgenoten niet komt (van der Have 1984, Swennen 1984, vermoedelijk omdat het voor de meeste individuen vanuit een afweging van voedsel tegen veiligheid niet loont om daar te foerageren. Ook hier lijken inferieure foerageervaardigheden en verschillen in competitieve kracht een rol te spelen (Swennen 1984, Cresswell 1994, Cresswell & Whitfield 2008. ...
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The recovery of Peregrine populations in the last few decades triggers the question what impact these raptors have on shorebirds using the Wadden Sea as a staging or wintering site. To answer this question, we took advantage of results of a four-winter study on raptor prédation on shorebirds on the Banc d'Arguin, a key wintering site for shorebirds on the East-Atlantic Flyway. We suggest that in the Wadden Sea, as on the Banc d'Arguin, populations of wintering shorebirds are not regulated through direct consumption by predators. Instead, raptors may have a profound intimidating impact on (groups of) shorebirds, which induces the latter to engage in a wide spectrum of anti-predation behaviours and body composition changes. This means that shorebirds pay predation costs by means of foregone opportunities affecting their long-term survival and reproduction, rather than by direct mortality. In a predator-prey game of anti-predation measures against stealth, predators will try to exploit vulnerabilities of their prey. Because of this, especially inexperienced birds or migrants which are unfamiliar with the habitat are prone to being depredated. However, also fattening (spring) migrants may suffer considerable predation costs, and these may increase in the near future, as growing numbers of Peregrines breed in the Wadden Sea region.
... One states that yearlings are forced to use suboptimal habitats due to competition with adults. This is the commonest explanation for age-specific habitat partitioning in birds, although evidence is based on observations rather than experiments (Gauthreaux 1982, van der Have et al. 1984. A second hypothesis suggests that yearlings and adults are specialized in their use of habitat types. ...
... However, to obtain a representative estimate, there are issues to address, such as: do the adults and young birds spend the non-breeding season together and are there any biases associated with the catching method? There are examples of age segregation across sites within a wintering area (Summers et al. 2005, van der Have et al. 1984). Therefore, one needs to take care when combining data from different sites, and weight the data accordingly. ...
... In fact, an extra energy expenditure on such flights may represent a heavy burden in the tight energy budget of shorebirds during the winter (Rehfisch et al., 1996) and may even affect their survival (Rehfisch et al., 1996;Drake et al., 2001;Durell et al., 2005), particularly in the case of small species, such as the dunlin (Piersma et al., 1993). Previous works with dunlins (Have et al., 1984) and oystercatchers (Haematopus ostralegus) (Swennen, 1984) in the Wadden Sea showed that higher status birds tend to roost closer to the best feeding areas than lower status birds, which also suggests a preference for minimizing the distance between roosting and feeding areas. ...
Article
Shorebirds are declining all around the world, mostly due to deterioration of the estuarine habitats used in winter and migration. Estuaries cover small areas, so it is essential to guarantee that shorebirds can access all the tidal flats where they usually feed at low-tide.Studying use of space by dunlins (Calidris alpina) in the Tagus estuary (Portugal), we noted that lack of suitably located high-tide roosts can limit the access of shorebirds to feeding habitats. Density of dunlins on foraging areas declined significantly with distance to the nearest roost, and fewer than 20% individuals foraged more than 5 km from two roosts where they were dye-marked.So to permit full access to feeding areas it is important to maintain a network of suitably located high-tide roosts. We developed a GIS modelling methodology to evaluate the adequacy of existing roost networks, and to estimate the consequences of losing or creating new roosts. The methodology requires maps with the location of roosts and foraging habitats, and knowledge of the distances that birds are willing to fly to reach foraging areas. It quantifies the proportion of foraging areas close to the existing roosts and the average distance that birds have to fly to reach potential feeding sites.Applying this methodology to the Tagus estuary we concluded that lack of roosts probably explains why the intertidal flats in the north-west of the estuary are underused by shorebirds. A modelling exercise suggested that this gap could be eliminated by creating a roost in an old drained wetland area. We also modelled the impact of the loss of two roosts that are currently threatened. Without them almost half of the available feeding areas will be too far from roosts to be efficiently used by dunlins, and possibly by other shorebirds.
... Preliminary indices on the numbers of Oystercatcher, Grey Plover and Knot, based on data from Texel, Terschelling, Ameland, Balgzand, and the Friesland and Groningen mainland coast (data IWRB Wader Research Group, largely supplied by counts organized by P. Zegers) juveniles often use different areas. This phenomenon has also been demonstrated in the Wadden Sea (van der Have et al., 1984; Swennen, 1984). The annual chick production can be highly variable between years (Boere, 1974; Furness & Bailhe, 1981; Summers & Underhill, 1987), which may be important information when counts reveal a dramatic drop in numbers of certain species (cf. the example for Knot on p. 374). ...
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Over the past 20 years, a tradition has developed in western Europe of carrying out shorebird surveys in estuaries. These counts are mostly conducted by amateur ornithologists, usually well acquainted with the area in which they are active. The results of these surveys are well reproducible for at least the abundant species, and information is available on the size of the errors which occur. In several parts of Europe, shorebird surveys have been carried out long enough to allow trend analysis of population size. Four examples from Great Britain demonstrate that monitoring schemes may also be used for management purposes, especially if combined with specific research projects. Long-term shorebird surveys can be used not only for simple analysis of trends in bird numbers, but also to provide a deeper understanding of the functioning of the ecosystem. They may also act as a tool to document the effects of management activities in the area concerned. Shorebird surveys in the Wadden Sea have not only revealed the extremely large importance of the area, especially for waders, but also show that different areas are exploited in different ways. They have also provided data on changes in bird numbers throughout the year. Apart from the counts carried out in January, the frequency of counts in most areas in the Wadden Sea has been too low to allow the results to be used for monitoring purposes. In an area as large as the Wadden Sea, some sites, often supporting large numbers of birds, may not always be covered during simultaneous surveys. This may seriously affect the completeness of the count. The best way to overcome this problem is to calculate index values, using only those sites which have been surveyed in successive years. Two monitoring schemes are proposed which may be effective in providing relevant information for management purposes.
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Individual feeding specialisation in shorebirds is reviewed, and the possilble mechanisms involved in such specialisations. Any specialisation can he seen as an individual strategy, and the optimum strategy for any given individual will be conditional upon its specific priorities and constraints. Some specialisations are related to social status and some to individual skills. Some are also probably frequency-dependent. However, most shorebird specialisations are constrained to a large extent by individual morphology, particularly bill morphology. For example, larger birds are able to handle larger prey, and birds with longer bills are able to feed on more deeply buried prey. Sex differences in bill length are uncommon in the Charardriidae, which are surface peckers, but are common in the Scolopacidae, which feed by probing in soft substrates. Sex differences in bill morphology are frequently associated with sex differences in feeding specialisation. There is evidence that different feeding specialisations are associated with different payoffs, in which case the probability of failing to reproduce or of dying will not be distributed equally throughout the population. I consider the population consequences of such feeding specialisations, particularly the different risks and benefits associated with different habitats or diets. I also consider the way in which individuals may differ in their response to habitat loss or change. I suggest that population models designed to predict the effect of habitat loss or change on shorebirds should have the ability to investigate the differential response of certain sections of the population, particularly different ages or sexes, that specialise in different diets or feeding methods.
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