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

valves may vary by a factor of two during the course of
the year (Cayford & Goss-Custard 1990, Zwarts 1991,
Zwarts & Wanink 1993). there is an opportunity to test
whether seasonal changes in the profitability curves
accurately predict a shift in size selection. Cayford &
Goss-Custard (1990). for example, analysed the seasonal
variation in the size selection for Mussels on the
Exe estuary. SW. England. The birds actively selected
those si/e classes predicted io maximize intake rale lot
mosl months of the year. However, in spring, the birds
fed on smaller Mussels than were predicted from their
profitability and availability. Cayford Ac (ioss-Custard
(1990) suggested that size selection may be influenced
at certain times of the year by still unmeasured differences
between size classes in prey quality and energy
content.
However, other apparently contradictory results
can be reconciled more easily with the optimal foraging
model. Handling time does not increase as the
flesh contenl s>\~ a bivalve increases because the time
taken actually to eat the llesh is only a small fraction of
the total (Wanink & Zwarts 1985). Most of the handling
time is spent in lifting the bivalve io the surface,
stabbing or forcing the bill between the valves, or hammering
a hole in the shell. Hence the profitability of a
given prey size may be as much as twice as high in
early summer as in winter. The rejection threshold,
therefore, should be lower in summer than during the
test of the year (Fig. 8). In fact, in Macoma. the reverse
was found (Fig. 4B), as Oystercatchers were more selective
in summer than at other times of the year. However,
all data on si/e selection in early summer refer to
breeding Oystercatchers which have the rather high intake
rate of 3 mg s ' (Ens el al. 1992, Bunskoeke el al.
1996). Such a high rate should have the effect of raising
the rejection threshold for Macoma. thus countering
the opposite effect on the rejection threshold of an
increased prey condition (Fig. 8). Thus, in this case,
two tendencies may be working in opposite direct ions.
The intake rates of Oystercatchers. measured over
periods of several hours usually vary between 1 and 3
mg dry llesh s' 1 (Fig. 13). When they feed al a rate of 1
mg s ', they would be predicted to accept all Scrobicularia
or Mytilus larger than 15 mm long, whereas when
the intake rate is three times higher, the rejection
threshold should be raised to 20 and 25 mm. respectively
(Fig. 10). The simple way to test these predic­
PREY SIZE SELECTION AND INTAKE RATE
168
tions would be to compare observed rejection thresholds
with the lower thresholds predicted from the intake
rale. There is. however, a methodological problem.
In the field studies summarized in Fig. 13. intakerate
is closely correlated with the size of the prey taken
(see below): the expected dependence of the lower si/e
threshold on intake rate may thus be attributed simply
to size selection itself.
The intake rate is determined by a combination of
three variables: the searching time, the handling time
and ihe dry llesh weighl of ihe prey taken. The search
time primarily depends on prey density (Hulscher
1976, 1982. Sutherland 1982b. Wanink & Zwarts
1985). Both handling time and prey weight increase
with prey size (Fig. 9), whereas there is a seasonal variation
in the llesh content of individual prey. The major
factor determining iniake rate in the reviewed studies
was the si/.e of the prey (Fig. 14); for instance. 59' I oj
the variance in intake rate on Scrobicularia could be
explained by prey size. A covariance analysis performed
on the 197 studies which measured intake rate
(Zwarts el al. 1996b) revealed lhat prey weight explained
16% of the variance and lhai both Ihe prey
species and season had significant effects (R 2 = 0.24
and 0.19. respectively). When prey of different
species, but of similar size, were compared. Cerastoderma
and Macoma were shown to yield a higher intake
rate than the other bivalves. The reason is that, for
a given size class. Cerastoderma and Macoma are
more profitable. This is due to their more globular
shape (Zwarts & Blomert 1992). so that they contain
more llesh than corresponding size classes in the other
three bivalves (Fig. 10).
The increase in iniake rate with prey size (Fig. 14)
is thus due to the high profitability of the larger size
classes (Fig. 10). The yield of the smallest prey taken
(Fig. 10) is below the average iniake rate of 2 mg S '.
So even if the birds could eat these small prey continuously,
without spending any time in searching for
them, the average intake rate would fall to the low level
of 1 or 2 mg s'. In contrast, the larger size classes can
be handled at a rate of about 10 mg s" 1 (Fig. 10). enabling
the birds to attain an intake rale of 2 mg s'. even
though as much as 80% of the feeding time is spent in
searching. In fact, the searching lime must have been
less than this, as Oystercatchers taking large bivalves
have an intake rate of 3 or 4 mg s ' (Fig. 14).