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prey density
II III
Fig. 12. Frequency distribution (% I of depth classes in the Oyster-
catcher's diet al font deusiiy classes. 6+ 12 + 24 cm ' (ll. 44 + 88
m : 1111: 175 + 262 in • I III I and 350 * 437 m ; 11V i. Irom Fig. 11
we have calculated which depth classes ihe bird should lake lo ob­
tain Ihe maximal intake rate (dark grey I and which classes ji might
add io the diei i grey i before reducing lis intake rale by more than 5%
(white).
Observed depth selection and intake rate
Figure 12 shows the observed depth selection for four
prey density classes. As predicted, the bird look all
depth classes at the low prey densities and ignored the
deep-lying prey when the density was high. The bud
did not show exactly the predicted change in rejection
threshold. However, the deviation is small and. as
shown in Fig. 11 A. deviation of 2-3 cm from the predicted
depth selection limit hardly reduces the iniake
rate. The bird added in fact only those depth classes to
ihe optimal set by which ihe intake tale was reduced
not more than 5' I.
The predicted intake tale lot the bird selecting exaclly
ihe optimal set of deplh classes Ills quite well
wilh the obsened values for the five lower prey densities
(Fig. 13). However, from density 175 m : onwards.
the number of prey taken by the Oystercatcher rises
above the predicted intake rate.
The predicted iniake rale was based on ihe assumption
that all prey must be laken from the optimal set of
depth classes. However, by using as second selection
criterion -ignore prey which are closed or noi gaping
enough, and so shorten the cutting time- the Oyster
OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE
149
catcher vv as able to reduce the handling time and so increase
its intake rate at higher prey densities by as
much as 5i)'. of the expected value. A density-related
selection of shells which could not be Stabbed immediately
has already been suggested by Hulscher (197(1)
foi an Oysieicalchei feeding on Cockles all luiiicd al
the same deplh.
We suggest that al prey densities below 175 in ; optimal
depth selection is the only decision rule, but
above that density selection on depth as well as on
easy' bivalves is optimized.
Discussion
Rate of discovery a
The disc equation of Holling (1959) is attractive because
of its simplicity. The intake rate depends on two
factors only, which should both be constant for all prey
densities: the handling lime and ihe rale of discovery a.
Since a equals XI) ' (cf. Fq 2). a will be density-independent
only if the search time increases n' limes
when prey density increases n limes. This was the casein
the "touch experiment' of I lolling (1959) but in the
'sound experiment' described in the same paper, a decreased
when density wenl up. Holling solved this
problem by dividing ihe search lime into two compo-
prey density (n-m-2)
Fin- 13. Relationship between intake rale and prey density. Pre­
dicted values i • rare compared lo ihe mean values, taerved
CL. (O).