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

their gut is full? Do they reduce their intake rate to the
level of the processing rate of 0.66 mg s ' or do they
stop feeding and start again when the alimentary tract
is partly or completely empty? Apparently Oyster
catchers do not lower their intake rale when the alimentary
tract is full, since intake rates as low as 0.66
mg s ' are rarely recorded. Instead. Oystercatchers rest
for a considerable part of the low water feeding period,
just like Whimbrels Numenius phaeoptis eating
fiddler Crabs Uca tangeri (Zwarts & Dirksen 1990).
Active foraging is often restricted to the first and last
hours of the low waler feeding period (Brown &
O'Connor 1974. Swennen et al. 1989). The higher iniake
rale on the incoming tide recorded in captive birds
by Swennen et al. (1989) and in the field by Zwarts &
Drent (1981) and Goss-Custard ct al. (1984) probably
guarantees that birds arrive at the high water roost with
a full gut
We have shown that Oystercatchers continuously
make feeding decisions lhat enhance their intake rate.
At the start of the chapter, we assumed they do this in
order lo minimize feeding time and thus maximize the
lime they can spend on other activities such as preening
and aggressive behaviour. However, when the processing
rate is usually so much lower than the intake
raie. ihe birds can preen or be aggressive during their
inevitable digestive pauses. This implies that feeding
and. for example, preening are not necessarily competing
activities. The question, already raised by Kersten
& Visser (1996a) is this: why should we continue to
expect Oystercatchers to always try to maximize their
intake rate and thus minimize the lime spent feeding.'
Maximizing intake rate would only seem to be relevant
when the birds have difficulties in achieving an iniake
rale of about 1 mg S"'; only in these circumstances is
the total consumption determined by the intake rate itself
(Fig. 15). It seems unlikely to be important for
Oystercatchers to attempt to increase their intake rate
PREY SIZE SELECTION AND INTAKE RATE
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and when ii has already reached levels of 3 or 4 mg s' 1
and vv hen the only apparent consequence ol doing so is
that they nuisi pause earlier to allow for digestion.
While these arguments seem to apply to many situations
in winter, they do noi do so in the breeding season.
The amount of food brought to the young often
limits reproductive success {Ens etal 1992). Breeding
birds have to feed in a hurry to return to their nest
and/or defend their territory or young, and the rate of
provisioning is not limited by the capacity of their own
gut to process food. Indeed, breeding birds feed at
higher average rates than non breeding Oystercatchers.
which are not lime-stressed (Zwarts & Drent 1981.
Hulscher 1982. Ens et al. 1996c). Moreover. Oystercatchers
were able to handle prey faster and to increase
their intake rales when they were experimentally
forced to collect their food over short feeding periods
(Swennen el al 1989). li seems that Oystercatchers do
not normally Iced at the highest possible rate they can
attain, even though they usually seem to make the most
profitable choices in the sizes of prey they select. The
task now is to investigate the trade-offs thai the buds
presumably make under various conditions. One such
trade-off might be to balance a higher intake rate
against the need to minimize the risk of damage to the
bill when breaking into their large and well-defended
prey (Hulscher 1996). This would mean that the longterm
advantage of an undamaged bill would be set
against the short-term goal of a higher food intake rate
and reduced feeding lime. Another trade-off might be
between a lower intake rate but a reduced chance of
being infected by parasites (Goss-Custard el al.
1996a). What is clear, however, is that a model in
which maximizing the intake rate is the only consideration
is not adequate. More sophisticated models,
w Inch include trade-offs between a variety of changing
influences, including the state of the bird's body reserves
(Stephens & Krebs 1986). are now required.