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

they mav take prey ol relatively low value because
thej select open Cockles. First. Cockles in poor comli
lion might be least able to shut their valves firmly
when attacked. Second. Ov stercatchers feed at low water
when only a minority of Cockles do not close their
valves i Iwisk 1986). It is to be expected that il is predominantly
those Cockles with low energy reserves
thai feed during the risky low water period.
The significance of our findings is that attempts to
measure the dailv intake rale of predators in the field
may overestimate the energy consumed if the predators
select relatively lean prey. Both Wanink (1992)
and this paper demonstrate that the resulting overestimation
of the food intake can be prevented by
analysing the predator's selection criteria and determining
the weight of prey likely to be taken. In some
cases any selection for lean prey can also be tested directly
by the analysis ol prey remnants (e.g. Fit/Gibbon
Ac I anshawe 1989). Eutermoser (1961) found thai
man named falcons took twice as many diseased or injured
crows as were shot by man. namely -IO'-; against
21'.. Health) crows weighed 580 g compared with
490 g for the debilitated ones. Kenward (1978). also
using the falconry technique, showed that the real
weight of captured prey was 15'.,' below the weighl of
the average pigeon in the population. An even larger
discrepancy was found by Bijlsma (1990) who studied
large falcon species preying upon Dunlin Calidris
alpina. The average weight of birds in the population
was 38 g, but the falcons caught juveniles which were
extremely lean, some 26*51 below the population mean
weight.
Predation risk and body condition
fhe depth to which a deposit-feeding bivalve Scrobicularia
buries itself is a compromise between starvation
and predation, and the compromise is different for
each individual (Zwarts 1986). It is risky to be near the
surface but by being there the animal enlarges ihe feeding
range of its siphon on the surface. Siphon weighl
has a cleat effect on burying depth, but if animals with
the same siphon weighl are compared, only Scrobicularia
with a small siphon and in poor body condilion
should risk leaving the depth refuge to improve their
ACCESSIBLE PREY ARE OFTEN IN POOR CONDITION
136
feeding eircumsianees. Apparently animals with large
energy reserves are able to survive a period of starvation
and at the same time the siphon can grow.
Recently many papers have documented trade-offs
that animals make between the risks of starvation and
predation. There are a number of behavioural changes
lhal reduce the risk of predation: an increase in scanning
at the expense of feeding (Lendrem 1984, Oliick
19871; a restriction of Ihe total feeding lime (Brown et
al. 1988. Lima 1988); a change in habilal use (Stein
1977. Grubb & Greenwald 1982, Sih 1982, Cerri &
Fraser 1983. Kramer et al. 1983. Werner et al. 1983,
Holbrook & Schmitt 1988. Pitcher et al. 1988.
Schlosser 1988) or a change in ihe diet (Dill & Rraser
1984. Lima & Valone 1986. Brown etal. 1988). Somepapers
have either argued theoretically or showed empirically
that animals devote more lime to predator
avoidance when they are satiated (MeC'leerly 1978.
Heller & Milinski 1979. Krebs 1980. Lima & Valone
1986. McNamara& Houston 1986, Weissburg 1986).
Thus hungry fish flee less quickly from a predator than
satiated ones (Dill & Fraser 1984. Morgan 1988): the
vigilance of birds may decrease when they are hungry
t Krebs 1980, Whitfield et al. 1988): birds with low energy
reserves are more likely to feed during risky periixjs
(Lima 1988): and birds may take more risks when
their energy demands increase, e.g. due Io premigration
fattening up (Metcalfe & Furness 1984).
II starving animals do lake more risks, predators
would be expected to lake prey with low energy reserves.
Along with the results presented in this paper.
there are two other reasons to think that such prey are
more vulnerable to predators. First, Starving animals
are easy lo catch because of their reduced speed le.g.
Yin & Blaxter 1987. Bijlsma 1990). Second, lean and
subordinate individuals i Murton et al. 1971) feed more
often in risky areas or during risky periods compared
with dominant conspecifics (Fretwell 1972. Barnard
1980. Schneider 1984. Hegner 1985. dc Laet 1985).
Therefore we conclude that the selection of relatively
lean prey by predators is a widespread phenomenon,
which certainly deserves more attention in studies attempting
to quantify the intake rate of predators in the
field.