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

PREY DEPLETION BY OYSTERCATCHER AND CURLEW
HOW OYSTERCATCHERS AND CURLEWS
SUCCESSIVELY DEPLETE CLAMS
Oystercatchers and Cm lews ignore prey which are unprofitable, i.e. those of which the handling efficiency
is below the intake rate during feeding (handling + searching) as predicted by optimal foraging
theory. The predaiion risk is maximal for clams Mya arenaria which are about I cm above the lower acceptance
threshold. Bigger clams arc laken less often since they live out of reach of the bill. Given a size
class, clams which are buried less deep have a greater risk of being taken. The resource partitioning between
both bird species is quite complete in the natural situalion. Manipulation of ihc food slock, by
planting big clams at a shallow deplh, elicited interference between Curlews and Oystercatchers. which
normally is very rare. Male Curlews take a few clams only, whilst it is a main prey for the females. It is
suggested that because males have a shorter bill than females, the proportion of accessible' clams above
the lower acceptable si/e limit is too small. Oystercatchers deplete the clams in the winter following the
second growing season. After this the Curlew females exert a heavy predaiion pressure on the remaining
clams.
Introduction
The leading problem tackled in most studies of the
feeding ecology of coastal wading birds is to what degree
the food supply on the intertidal Hats is a limiting
factor for the bird populations (see review by Goss-
CllStard 1980). Their food supply -mainly macrobenihie
animals living in the substrate- is highly variable
from season lo season and also from year to year. The
study by Beukema (Beukema et al. 1978, Beukema
1979) of the intertidal flats of the Balgzand (western
Wadden Sea) shows thai the spatlall of important
shorebird prey species like the clam Mya arenaria. die
Mussel Mytilus eduhs and the Cockle Cerastoderma
edule is very erratic. Their survival over the winter period
also varies from year to year.
The variability in prey density is even larger if we
take into account the lluetuaiions in the numbers of
prey which are likely to be taken by the different wading
birds; some of the prey are ignored because they
are too small to be profitable, whereas others lie too
deep in the substrate and are out of the reach of the
waders' hills (Reading & McGrorty 1978).
We summarize in this chapter some results of cur­
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rent research on colour-banded birds feeding on the
mudflats along (he Frisian eoasl. near the village of
Moddergat. We will explain why Oystercatchers
Haematopus ostralegus and Curlews Numenius ari/ttata
feeding on clams select different size and depth
classes and will show that both bird species combine lo
deplete the food stocks in the course of 2 or 3 years.
Selection of the profitable clams
Not all prey accessible to birds are in fact laken. For
example. Herring Gulls LOTUSaigeniams ignore Shore-
Crabs Carcinus maenas below c. 20 mm carapace
width, whilsi the rejection threshold for the Curlew is
c. 10 mm, and for the Redshank Tringa totanus and
Greenshank Tringa nebultiria it is still lower (c. 5 mm)
(Zwarts 19X11. It w; -led by Viae Arthur & Pianka
(1966) that the lower acceptance threshold was
chosen by the bird in such a way thai a maximal iniake
rale was ensured. Prediction of the lower limit for Oystercatchers
feeding on Mussels was correct (Zwarts &
Drent 1981). and the data Hulscher (1982) for Oystercalchers
preying upon Macoma balthica also support