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

the rule: the observed lower limit laken. of c. II mm
shell lengih. is near the expected acceptance threshold.
The applicability of this rule to our study of wadei S
feeding on the clam Mya arenaria can be investigated
by comparing the handling efficiency (mg ingested per
second of handling) of each size class of prey with the
overall intake rate (mg ingested per second of feeding),
where feeding = searching + handling. Optimal foraging
ttieory predicts that prev sizes for which Ihe handling
efficiency is below the average rate of focxl iniake
should be ignored (see Krebs 1978 for a general
review of the topic).
We observed lhat Oystercatchers feeding on clams
did not lake shells below a certain size. Since it is difficult
to measure the prev size exactly in a field situation,
we decided to capture some Oystercatchers and
offered them clams of different size classes in controlled
experiments, as Hulscher had done with Macoma
balthica.
In one ol" the experiments an Oystercatcher fed on
clams of different size classes buried in the substrate.
all at the same depth, just below the surface. The yield
per second of handling time appeared lo be maximal
ell of Id 4S mm (Fig. I).
Hulscher and his students made a series of observa­
PREY DEPLETION BY OYSTERCATCHER AND CURLEW
tions in October and November 1980 on free-living
Oystercatchers feeding on clams in our study area.
They measured the handling ol about />(KX) prey and
collected the fresh shells recently opened by Oystercatchers.
They found that clams of 15-40 mm (average
28.2 mm) were taken il Ig. 2. left panel). Bui in contrast
to the captive bird, the free-living Oystercatchers
We were able to locale the clams seen In be taken in relation io a grid of thousands of numbered poles, separated from each other h> 15 in.
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