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

PREY DEPLETION BY OYSTERCATCHER AND CURLEW
T-S*I 1 r?p-r-o-ro-i 1 1— —r-*Tf i 1 1 rr-
0 1 ^ 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10
size of Mya arenaria (cm)
Fig, 2. A. Si/e selection by Oystercatcher (left) and Curlew (right), preying on clams ('.; frequence distribution per 5 nun class; .ample si/c
indicated). Clams ealeii hy free-living Oystercalchers in Octobcr-Novemher I9K0 or Curlews in Augusl-lVcemher 1978 are compared to the
clam population on oiler in the Moddergat area for Oystercatchers and in the Nes area foi Cuilews. The righl panel shows ihe prey selection hy
a single colour-handed Curlew (20Y): the size has heen derived from (he known relationship between handling time and weighl (and thus size)
of the clams. B. Risk to a clam of being taken, relative to ihe maximal predaiion risk ol a single si/e class The maximal risk is set to I (Oys­
ieicalchei: clam si/e 3-4 cm. Curlew: clam si/e 4-5 cm). The relative risk is derived Irom the data given in Ihe upper panel. The solid triangle
denotes the predicted lower limit and Ihe open triangle ihe mosi profitable si/e class (see lie. 11.
did nol take all the llesh from ihe shells. In man\ cases
the) pulled out the siphon, together with the greater
part of the body, in one jerk. On average 22% of the
llesh remained in the shell and the flesh taken averaged
51 mg ash-free dry weight. The mean handling time
was 5.82 s. so the average yield during handling was
8.74 mg/s (open square in Fig. IC). This field estimate
is quite close to the peifonnance curve ol" die captive
Oystercatcher.
Since free-living Oystercalchers devote 30.3*3S "I
(heir feeding time to handling the clams (the other
69JVr being spent in searching for prey), the average
rate of intake of biomass amounted to 0.303 x 8.74 =
2.65 mg/s (shaded block in Fig. IC). The theory predicts
that all prey sizes yielding a lower rate of biomass
intake would be ignored. The smallest clam given to
338
the captive bird was 22 mm. The handling efficiency
for this is still above the predicted lower threshold. For
clams between 20 and 40 mm there is a linear relationship
between yield and shell size. Extrapolating the
line downwards, we would expect that clams of |7.3
mm and smaller should be rejected, which fits well
with the observed threshold in the free-living Oystercalchers
ll-'ig. 2).
The clam is also an important prey for Curlews, but
the threshold size required for acceptance is higher.
For this species we measured the profitability of different
shell si/cs I'm licc-lniiig Curlews. It was easy to
measure the handling time but impossible to estimate
directly ihe size of the shells taken, because a Curlew
pulls the siphon and pieces of flesh out of the shell,
from below the mud surface. It was possible, however.