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

1 2 3 4 5 6 7
probing depth (cm
PREY SIZE SELECTION AND INTAKE RATE
350 prey nv 2
0 1 2 3 4 5 6 7
maximal prey depth accepted (cm)
Fig. 12. Experiment toshow why an Ov siercatchei ignored deep, less profitable prey when the intake rale increased due to a high prev density
V. Tune needed to handle Scrobicularia 37-38 mm long which were laken from different depths (s ± SE). B. Time needed io probe to different
depths (s ± SE). C. Predicted feeding rate (Scrobicularia min ') at two prey densiiies when an Oystercatcher took prey from the upper I. 2. ...
7 cm of the substrate. Actual observed depth selection is indicated vv ith horizontal bars. As predicted, the bird took prey from all depths when
the prey density was low and only shallow prey when the prey density was high However, the bird did this al a much higher rate than predicted
(explanation given in text). Data of Wanink & Zwarts (Iis s >
nomics of foraging are being calculated.
It has been shown experimentally that a captive
Oystercatcher decreased its probing depth from 7 to 3
cm when the density of the prev on offer. Scrobicularia
36-37 mm long, increased from 24 to 350 prey nr 1
(Wanink & Zwarts 1985). It took more time to handle
deep-living prey (Fig. I2A). hence prey profitability
decreased with depth. Moreover, it also took more time
in the first place to locate a prey at greater depths (Fig.
I2B). The encounter rate, defined in optimal foraging
models as the inverse of the searching time, for prey at
different depths could be calculated from the effective
touch area and the probing time at each probing depth.
Since the encounter rate and the handling time were
both known for each depth class, the optimal set of
depth classes, which should be included in the diet to
maximize the intake rate, could be predicted exactly
with a multi-species functional response equation
(Charnov 1976) (Fig. 12C). The depth selection made
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by the bird was very close to that which was predicted.
The Oystercatcher took prey from all depths when the
density was low but rejected the deep, less profitable
prey when prey density was high: by doing so, it increased
its intake rate at high prey density by eating
only the most profitable, shallow prey. It may be
thought from Fig. 12C that the bird could have done
slightly better by selecting prey only from the upper 1
cm, and not from the upper 3 cm. as it actually did.
But. in fact, the Oystercatcher did even better than predicted
by using a second selection criterion, as will be
explained in the next section. The conclusion from this
experiment is that Oystercatchers are indeed able to
vary the rate at which they encounter different prey
types and. in doing so, increase their intake rate.
Rejection of prey to increase intake rate
The previous section showed that Oystercatchers are
able to adjust their searching behaviour in order to in-