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

FOOD SUPPLY HARVESTABLE BY WADERS
S'O'N'D
Fig, 10. Seasonal variation in the accessihilitv „\ Mamma hallluca
(15 mm) over 7 years The proportion of the population living in the
upper 2.4 and 6cm is sh.vv n
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variation in condition of the prey actually taken by
buds mav be even larger ih.m indicated bj the mean
condition for the population as a whole. The condition
of Scrobicularia vanes with burying depth: the
condition of shallow bivalves is about half of those that
live more deeply and the same has been found in other
bivalves and in \ereis. Ihe consequence is thai onl\
lean prey are accessible in winter and thai Oystercatchers
feeding on Scrobicularia. for instance, face a
seasonal variation in the llesh weight of the prey they
actually take, which is 1.3 times greater than ihe
variation in the entire population (Zwarts & Wanink
1991). Since most studies summarised in Table 2 have
not taken this into account, the feeding conditions in
winter are even worse than indicated.
It should be noted that Table 2 expresses the intake
rale in terms of dry flesh, because energy density was
rarely measured. Since our study showed that the
energy density oi Scrobicularia and Mya. is III'.
lower in winter than in summer (Fig. I), the seasonal
difference in intake rale is even larger in these two
species when expressed in terms of energy, me critical
quantity.
Feeding waders must make many decisions: where
to iced, for which prey species to search and which
size class to select. These decisions can only be
Understood if the prey that are actually available are
known and their profitability measured. In view of this,
the next three sections attempt to define several aspects
of prey availability in waders. After dealing with prey
profitability, the results from all four sections are
considered to describe ihe fraction of the prey thai is
harvestable. Finally, all this information is used to
analyse prey switching in waders and to discuss lo
what degree the distribution of waders over the wintering
areas is related to a deterioration of their
harvestable food supply between late summer and
w inter in the tidal Hats in NW. Europe.
The accessible prey fraction
Benthic prey are accessible to waders only if they live
within reach of the bill. The accessibility of immobile
prey (benthic bivalves) will be discussed first, followed
by an analysis of the more complex situation of
mobile prey (e.g. worms). Although bivalves are
capable of changing their position in the substrate, the
attachment of very thin nylon threads to Macoma.