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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
Mortality due to Starvation implies an exhaustion of
the nutrient reserves. The feeding circumstances in our
siudy area were poor between autumn 1979 and spring
1982. compared to the lime before and after. However,
the body weight of ihe Oystercatchers in these four
years appeared not to deviate from the long-term average
monthly means i Fig. 231. Hence, the average Oystercatcher
was. even in these winters, able to satisfy its
energy requirements. This seems inconsistent with the
higher mortality in the poor years (Fig. 22B). However,
as shown elsewhere (Zwarts el al. I996e). Oystercatchers
attempt to increase iheir body weighl as
soon as their fcxxl supply is less predictable. Thus a
higher bcxly weight is to be expected if the average iniake
rale is low. On the other hand, some marginal
birds will possibly not be able lo attain the required
higher consumption. II this is so. we would expect a
larger variation in the body weights within the population
in years with a poor food supply. The frequency
distribution of the bcxly weights in the three poor
years, however, appeared not deviate from those of the
other years. Possibly, the fraction of marginal birds
266
which run a high risk of dying in periods of low temperatures
and/or reduced daily consumption is too
small to be detected in the bird catches we made.
Our general conclusion is that there are large local
variations in the ftxxi supply harvestable to Oystercatcher.
but that the Wadden Sea. unless the mudflats
are covered by ice. offers enough fcxxl for the birds.
pro", ivied thai ihey either switch between prey species
and/or between areas This is probably generally true
for waders in Wadden Sea. since Zwarts & Drent
(1981) and Zwarts et al. (1992) arrive at die same conclusion
in their work on Oystercatchers on .Schiermonnikoog
and Knot along the Frisian coast, respectively.
Nonetheless, the increase of winter mortality at a lower
food supply, suggests thai the winter remains a difficult
period for the Oystercatcher. as also found by
Camphuy sen ct al. (1996) who could show thai, over a
long series of 27 years, more Oystercatchers beached
when ihe food supply for Oystercatchers in the western
part of the Wadden Sea reached low levels
(Beukema 1993a i.