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

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size of Mya arenaria in cm
Kij». 4. Deplh of clams on offer in relation to their size, and depth of
84 clams taken by the colour-banded Curlew 20Y (dots). The grej
field shows ihe depth range at which 9Sfl of Ihe clams are living
(based on the same dala as in Fig 31
shows, large clams appear io be more profitable than
small ones. Mosl clams taken by the bird were indeed
large (Fig. 2A). The risk to a clam of being taken was
maximal if it measured 4-5 cm (Fig. 2B). but the risk to
the most common clam size present (2-3 cm) was 130
times less.
From Fig. IC it can be seen thai clams below 23.6
mm length would be unprofitable for this bird. In
March 1981, when large clams were very rare, we observed
that the marked bird look some small prey but
since we could not locate the clams which were taken.
their exact size was unknown. Assuming these were
solely second-year clams, which averaged 25 mm in
si/e at that time, the bird would have obtained 4.14
mg/s handling time (indicted with an open circle in
Fig. IC). which is just above the rejection threshold.
Certain other colour-banded Curlews also took
clams of about 25 mm. or even smaller, but in those
PREY DEPLETION BY OYSTERCATCHER AND CURLEW
10
340
cases ihe birds swallowed ihe ekim whole, including
ihe shell. The handling time was very short: 3.41 s (n =
252) for two Curlews observed in November 1980;
hence they obtained quite a high yield of*).: mg/s handling
lime. Eating clams in this way may have disadvantages,
however, for is was observed predominantly
in periods when big clams were very rare (the winter of
1977-78 and of 1980-81). and even in those lean w inters
musl o| the colour-banded Curlews for which
clams were the main prey never swallowed the small
ones. Perhaps they would have caused a "digestive bottleneck'
(Kenward & Sibly 1978).
The accessible fraction of the profitable elams
The data on the yields of food from different sizes explain
why Oystercalchers take smaller sizes than
Curlews, but cannot help us to understand why the
Oystercatchers on Ihe mudflats do not attack successfully
those clams of over 4 cm and why Curlew 20Y
took relatively few clams above 6 em (Fig. 2). These
upper limits are in fact determined by the proportion of
the different size classes within the reach of the hills of
the two waders.
To investigate the depth distribution of the elams in
each size class, we used a circular corer (of surface
area 176 cnr'i pushed 4(1 cm into ihe substrate. After
breaking open the core, it is possible to measure accurately
ihe distance between the mud surface and ihc
upper tip of die bivalves. From these measurements it
appeared (Fig. 3) that an Oystercatcher cannot find
clams of more than 4 cm in length in the upper 7 cm of
the substrule (the bill length of an Oystercatcher) and
thai the greater part of the shell above 7 em length arc
out of the reach of the female Curlew (bill length 14
cm). Even fewer are accessible toamalelbill length 12
cm) (Fig. 3, inseti.
Most clams taken by the marked Curlew lived,
given their size, al remarkably shallow depths (Rg. 4).
Nearly all were buried above the mean depth of their
respective size classes. The bird managed to find ihevery
rare clams of more than 6 cm length which lived
at atypically shallow depths, only 8-14 cm below the
mud surface.
In the summer of 1979 we manipulated the food
supply in the former feeding territory ol" the marked