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time during the feeding period. As a consequence,
there is no seasonal variation in the amount of food
consumed during an average low water period in daylight.
Discussion
Does profitability matter?
Predators cannot choose prey that are not available,
lor instance, bivalve prey are in any case not available
io Oystercatchers if they live out of reach of the bill.
Depending on whether bivalves are opened by stabbing
or hammering, prey may be defined as available if
the bill can be stabbed between the valves, or if the
shell is not too strong to hammer a hole in it. Yet. Oystercatchers
do not simply take all prev from the available
stock. As has been well documented. Oystercatchers
refuse small prey due to their low profitability
(reviewed by Zwarts et al. 1996a). For the same reason,
the birds may also select from the available prey
onlv the most profitable prey lhat are living at a shallow
depth (Wanink & Zwarts 1985), that have slightly
opened valves (Hulscher 1976. Wanink & Zwarts
1985) and/or that have thin shells (Durell & Goss-Custard
1984, Meire & Ervynck 1986. Sutherland & Ens
1987, Ens & Alting 1996a & 1996b. Meire 1996a &
1996c). Finally, as predicted by the optimal prey
choice model (e.g. Krebs & Kacelnik 1991). Oystercatchers
are more selective when their intake rate is
high (review Zwarts el al. 1996a): as intake rate rises.
Oystercatchers successively drop the least profitable
prey from their diet. For instance. Oystercatchers take
prey from the upper 7 cm of the substrate when their
intake rate is low but only from the upper 3 cm when
the intake rate is high (Wanink & Zwarts 1985).
What determines prey profitability?
I lav ing firmly established the importance of profitability
as a criterion for prey selection in the Oystercatcher.
we must enquire in more detail into the factors
that determine profitability. The data summarized in
Fig. 8 show (hat the profitability of prey taken by Oystercatchers
varies between 1 and 100 mg s '. i.e. two
orders of magnitude! A large part of this variation may
be attributed to the way in which prey, through their
defenses, are able to prolong the time the predator
PREY PROFITABILITY AND INTAKE RATE
197
needs to attack and eat them. For hard-shelled prey, we
may hypothesize that the decrease in profitability with
the degree ol armouring, depicted in Fig. 10, can be
explained hv ih,-extra time needed lo prepare and open
the prey. To explore this hypothesis, we musl break
down the handling nine inio its consecutive components.
First, Oystercatchers must recognize prev as edible.
However, it is likely that the time-cost of recognition
is so small lhat ii can be safely ignored. For
instance, Wanink & Zwarts (1985) found that detection
and rejection of pre) happened so quickly in Oyslercatchers
feeding on buried prey lhat it could not be
measured, not even with the aid of a high speed camera.
Thus, handling lime may be subdivided into three
significant components: (11 lifting and preparing time;
(2) opening and cutting time and (3) eating time
(Speakman 1984a. Wanink & Zwarts 1985. 1996).
What is known about the relative duration of these
components.'
(1) Lifting and preparing time. When prey located
in the substrate are lifted to the surface to be
opened, lifting itself lakes, on average, a quarter of the
total handling lime (Wanink & Zwarts 1985. 19%).
The handling tune is I 25-1.50 times longer when burrowing
prey, such as Macoma. Mya and Scrobicularia,
are extracted from the substrate rather than being eaten
in situ (Wanink & Zwarts 1985, 1996. Hulscher et al.
1996: see also Fig. 5). A soli-bodied prey taken Irom
the surface, such as the Leatherjackei. is handled at
least twice as fast than those which have to be extracted
from the substrate. Grasping or lifting time is
also zero in Oystercatchers that slab the bill directly
between the valves of Mussels and Cockles, or hammer
Mussels in situ on the dorsal side of the shell. In
contrast, the handling times of Mussels hammered on
the ventral side are relatively long because the Mussels
have to be lorn off the bed and turned upside down
(Cayford & Goss-Custard 1990; see also figs. 2 & 3).
(2) Opening and cutting time. Armoured prev
musl be opened by hammering or stabbing, alter
which the flesh can be separated from the shell. Opening
and cutting are absent in prey eaten vv hole, but it
takes about 2/3 of the handling time of bivalves, and
even more when prey are hammered. Before lifted bivalves
are opened, they are sometimes transported.
Ovstercatchers may walk for several seconds with
their prey to a site with a substrate firm enough to ex-