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

and maiiv are too thick-shelled to be opened (Kg, 2).
like slabbing Oystercatchers. hammering birds reject
Mussels below 20 to 25 mm long. bin. in contrast to
slabbers. Mussels larger than 60 mm are laken only infrequently.
When a correction is made for the prey
fraction thai is unavailable due to shell thickness and
barnacle cover (Fig. 2), the proportion or each large
prey class taken roughly coincides with their available
density (Meire & Ervynck 1986). or explains al least a
part of the deviation between observed size selection
and frequency distributioii of si/e classes on offer
(CayfordcS Goss-Custard 1990).
Size selection and optimal foraging
Predicted 'active si/e selection'
The random touch model tests the assumption that
birds lake prey al random. In fact, as the results in the
previous section show, the observed prev selection often
deviates from the predictions of the model. Oystercatchers
apparently prefer some si/e class to others.
Why? Prev size selection in Oystercatcher is analysed
here within the framework of optimal foraging theory
(Enilen 1966. Mac Arthur & Pianka 1966. Krebs &
Kacelnik 1991). The basic assumption is thai predators
are able to rank prey according to their profitability,
defined as ihe intake rate while prey are being handled.
They are predicted to reject prey for which the profitability
is below the current average intake rate over
both handling and searching combined. The decision
rule governing the rejection threshold is therefore
based on the relative profitability of handling compared
with continuing to search, i.e. whether the bird
can achieve a higher net intake rate by continuing to
search for more profitable prey than it could achieve
by handling a less profitable, although more frequently
encountered, prey. This leads lo the prediction thai.
when the profitability of the prey remains the same, the
rejection threshold should increase as ihe intake rate
increases. The rejection threshold should also increase
when the iniake rale remains the same but the profiiabiliiv
of all prey types decreases: lor instance, because
the prey are lean. The rejection threshold should
therefore be flexible within clearly defined limits, as illustrated
in Fig. 8.
PREY SIZE SELECTION AND INTAKE RATE
162
10 20 30 40
length of prey (mm)
intake rate
lotworwiil cangal
Fig. 8. The optimal pre) size selection model. The two slopes delimit
the seasonal variation in profitability (mg a ' handling) as a
Innclionol prev si/e I'rev lor which ihe prolil.ihililv is below Ihe illlake
rale line s' feeding, i.e. during both searching and handling)
should be rejected. Which pre) shook) he rejected thus depends on
the level oi the intake raie .is well a-on ihe length profitability slope.
The .lark shaded Held shows the expected range within which the
lower acceptance threshold should he lound when Ihe intake rale
varies between I and 4 mg s ' feeding.
Small prey are less profitable
Even after a correction has been made for the small
'effective touch area" oi small prey,Oystercatchers appear
lo lake fewer oi them than would be expected on
Ihe basis of the frequency with which they are encountered
(Figs. 2 to 7). The birds always completely reject
prey less than about 10 mm long in Macoma and
Cerastoderma and about 15 to 20 mm long in Scrobicularia.
Mya and Myiilus. There might be a very simple
explanation for this. Oystercatchers are specialized
to open hard-shelled prey before they eat the llesh. in
contrast lo Knot Calidris canutus or Bar-tailed Godwits
limosa lapponica which swallow ihe prey whole
and crush them in the stomach. There must be a size below
which Oystercatchers are hardly able to separate
the flesh from the shell. Whether or not this is close to
ihe observed rejection threshold has still to be tested.
On the other hand, several papers have used the
optimality approach to explain the size rejection
threshold of Oystercatchers. The prediction is that, below
a certain size threshold, prev are simply noi worth
taking since their energv value is too low given the
time required to handle them: i.e. to open them and eat
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