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a highly significant relationship between prey profitability
and si/e in all five bivalve species. This means
that profitability can be ranked simply according to
prey size.
Large prey are difficult and sometimes even
dangerous to handle
Not all prey that are attacked are actually taken, so
time is spent in failed attacks. When calculating the
profitability of a particular size class, this wasted handling
time has to be taken into account when calculating
the average time needed to eat them. The difference
this can make to the calculation of profitability
can be illustrated by hammering birds. Oysteicatchers
successfully hammering into Mussels spend more time
in breaking into a large one than into a small one. but
this is worthwhile as the flesh content is disproportionately
greater. But if the wasted handling limes are included
in the calculation, the profitability of hammered
Mussels actually decreases in Mussels larger
than 50 mm long (Fig. 11).
Another factor which falls outside the scope of the
simple optimality model being discussed here is the
potential risk to the bill of attacking large prey. Both
Sutherland (1982c) and Triplet (1989a) found that
larger Cockles are refused more often than small ones.
As the time lost is insignificant, the birds may be reducing
the risk that the bill tip will be damaged. After
being stabbed. Mussels may also close their valves
firmly on the bill, so that an Oystercatcher may eventually
die due to starvation (Hulscher 1988). But the
risk appears to be small and. with exception of Mussels
being hammered, the amount of lime spent in wasted
handling time is not large. The general conclusion
from the previous section that the prey size predicis
profitability is largely unaffected.
Oystercatcher can vary encounter rate
Another important consideration in calculating
whether a particular size class of prey should be taken
is the rate at which prey are encountered. Oystercatchers
can control encounter rates with prey through
changes in their search behaviour. For example, when
Oystercatchers switch from touch to visual hunting
(Hulscher 1976. 1982). they change from randomly
PREY SIZE SELECTION AND INTAKE RATE
165
20-
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di
E. 12f
•I"
l 8-
E
o h
4
20 30 40 50 60
length of Mytilus (mm)
Fig. 11. Profitability (mg ash-free dry flesh s' handling) as a function
of length for Mussels which are hammered on the ventral side.
Upper line refers to Mussels th.it were acluallv eaten so the nine COM
was just the handling time for those Mussels. The lower line also includes
all die lime wasted on other Mussels of the same size dass
which were iinsiicccsslullv hammered I Meire & F.rvynck I'INCI
probing into the mud to searching on the surface For
signs of the prey beneath (I lulscher 1996). This means
thai ihe encounter rate with potential prey has to be defined
according to the feeding technique used. Again.
Oystercatchers visually hunting for tracks or inspecting
bivalves on the surface may vary the encounter rate
with different prey types by varying search speed
(Cayford & Goss-Custard 1990). The speed at which a
foraging animal searches has been described as an
adaptation to the crypticity of the prey (Goss-Custard
1977a, Gendron 1986. Zwarts & Esselink 1989) and
observations on the walking speed of Oystercatchers
feeding on different prey types is consistent wilh this
idea i F.ns et al. 1996a).
Although randomly probing Oystercatchers provide
a good opportunity to measure encounter rates, ii
would be wrong to assume that they are fixed for a
given prey density and deplh distribution. This is because
Ov stercaichers can modify probing depth. As already
described, for example, Oystercatchers probe
twice as deeply when searching for the deep-living
Scrobicularia than when they feed on a shallow -iiv mg
prey, such as the Cockle. Oystercatchers must therefore
also make the decision on 'how deep to probe' and
this too must be taken into consideration when the eco-
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