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

2 3 4 5
probing depth (cm)
Fig. 7. Relationship between probing tunc and the depth of a probe.
The values shewn (means ±954 C.L.: sample sizes indicated) are
based on film analysis (resolution l;l(.siol searching behaviour U a
prey density of 0 in ": Im probing time) = 0.39 (probing depth)- 2.49
(n = 7. r = 0.97. p < 0.0051.
1 2 3 4 5 6
prey depth (crn)
Fig. 8. Predicted encounter rate (X) per depth class, for three different
prey densities iprey nr*).
the selected size class, we assume that the bird has to
probe /+1 cm to locate a prey in depth class /. P-loct depends
on D. only, because s is constant for all depth
classes. Dt is known, so Eq (5) can be solved, for t has
been measured using a film analyser (Fig. 7). The logarithmic
increase of probing time with probing depth
OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE
146
100 200 300
prey density (n-nr 2 )
400 500
Ug. 9. Observed mean encounter r.ue I#I t 959 CL. The while
lines represent values of >. ranging from no depth selection (upper
line) to selection ol depth class Ocm only (lower line). The solid linegives
Ihe predicted X for the bird selecting the optimal deplh classes
as calculated in Fig. 11.
determines Ihc way the predicted encounter rale decreased
for prey which were buried at greater depth
(Fig. 8).
Observed encounter rate
We never saw the Oystercatcher reject a prey it had
discovered ('negalive handling time"). Thai is why we
used ihe observed searching lime per prey taken to calculate
the mean encounter rate. However, analysis of a
high speed film showed that a prey can be located and
refused too quickly to be noticed by an observer. We
do not know how often we missed this kind of quick
encounter, bul ii is certain that the observed encounter
rate is an underestimation of me actual rale. Comparison
of the predicted encounter rate for a random
searching predator and the observed rate (Fig. 9) reveals
that at low prey densities ihe Oystercatcher took
all prey it found, whereas at high densities less prey
than expected were taken, perhaps because the observer
did not see encounters where prey were rejected.
Another explanation for the lower-than-expected
rate at high prey densities was suggested by the film