waders and their estuarine food supplies - Vlaams Instituut voor de ...

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FOOD SUPPLY HARVESTABLE BY WADERS
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Fig. 18. Variation in body weight (AFDW. mg) of Mytilus eduli.s 50
mm long 111 seven areas. Sources: Exe (Cayford & Goss-Custard
1990). Conwy (Dare & Edwards 1975: averages lor three years),
Moreeambe Bay (Dare 197?; average- Im three years; inrmiM!
has been made fot weight loss due 10 formalin (12.8%); the paper
gives DW; these were converted into AFDW. assuming thai ash
content is li'.i); Eastern Scheldt (Craeymeersch et al. 19X6;
corrected for weigh) loss due to lormalinl; l.vnlier A Caltewater
iH.ivne & Worrall 1980: weights arc ol Mussels t> cm long: these
were tranrfecmed to those of 3 cm using regressions given in iheir
paper).
content per prey starts to decline earlier in the winter. It
is not likely that the accessible fraction in winter differs
much when areas at different latitudes or average
sea water temperatures are compared.
ED
However, it does not follow thai, for all wader
species, ihe feeding conditions are always worse when
the ambient temperatures are low. First, the llesh content
of prey may be better: Macoma and Scrobicularia
lose more body weight in mild winters than in cold
ones (Zwarts 1991. Beukema 1992b). Moreover, there
are incidental occasions in winter, usually associated
with a large drop in temperature, when the benthic
food becomes highly available to birds. Arenicola and
Nereis may leave their burrows to migrate to deeper
water (Werner 1956, Verwey 1958. Dankers & Binsbergen
1984) and many worms become stranded and
can be easily picked up from the mud surface (Zwarts
unpubl.). Furthermore, during and after periods of
frost, moribund Cerastoderma with gaping valves
provide a rich food resource lor gulls and waders that
have remained (Zwarts unpubl. 1. Finally, during the
winter half of the year. Nereis use a feeding method
that makes them easy prey for birds. Whereas in
summer they mostly remain in their burrows to filter
food from the overlying water, in winter they feed
more frequently on the substrate around their burrows,
especially on sunny days in late winter and early
spring (Esselink & Zwarts 1989). This is why Curlews
feeding on Nereis achieve their highest intake rates in
late winter, and not in summer or aulumn (Zwarts &
Esselink 1989). The surface feeding by Nereis also
gives Redshank the opportunity to sw itch to Nereis at
low mud temperatures when the surface activity of the
preferred prey Corophium is already inhibited (Ooss-
Custard 1969).
Further research should focus on the profitability
and availability of benthic prey for wintering waders
along the East-Atlantic coast. This means thai il is
necessary to collect data on the body condition,
burying depth, surface activity and defecation rates of
the prey. Il will not be easy to show latitudinal variation,
however, since, as shown and discussed above, at
any one place large annual differences may be expected
in the profitability of the prey and in the fraction
available to waders.