Hydro-ecological relations in the Delta Waters

each of the birds' bill. Different age classes of burying bivalves.
such ae! Scrobiacularia plmb andMacma balthica. can be successively
eaten by different- species of waders depending on their bill ldngtb as
the older individuals lie deeper in the substrate (Zmuts and wdnk.
1984). Similarly dabbling ducks can reach aquatic plants only whitin a
small depth range. Mute Waas can reach down to 1.5 meter, Cwts oaly
to 0.5 meter. Bentbic feedins ducks are also limited by the water depth
as they can stay rmdet- water only for a certain time.
However, the availability of the prey population can also he dependent
on its own activity. Corophium volutator, a small crustacean Living in
If-shaped burrows, is detected by its predators, e.g. Redshanks. when it
is feeding on the mudsurface by quickly moving the antennae. After a
Bedshank passed, all C. volutator retreat in their burrow, due to the
vibrations in the sediment caused by tbe walking bird, aad become
unavailable to the predators for nearly a quarter of an hour (Gsss-
Custard, 1970).
gnother very important factor iufluencing bird distribution is
salinity, not only by its effects un the prey populations, but by its
influence on the osmoregulat1an of the predator. B i r h feeang on
marine prey organisms have to excrete the excess amount of salt intake,
while feeding, throngh the dasal glands. If the salinity of the water
where thep are feeding is too hlgh, they need to drink fresh mater in
order not to dehydrate (Nysrrtim and Pehrsson, 1988). It is very well
documented that the Pochard, which feeds on Eelgrass (Zostera marina)
in the saline Grevelingenmeer, rest on the fresh Haringuliet (Boudewijn
and Mes, 1986). Nystriim and Pehrsson. 1988, linked both distribution
and diet of swsrel coastal waterfowl species to their uptake and
ability to excrete salt. It is most probably that the very high
densities, especially of diving ducks, in the Veerse Meer, in
oompirrison to the Grevelingen Meer where the food supply i~ much
higher, might be due to the much lower salinity in the Veerse Meer.