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

able lo sw itch 10 oiher prey which guarantee an intake
rate high enough to get the necessary amount of food
within the restricted time the tidal feeding areas are exposed.
Generallv speakinc. Ihe Wadden Sea offers the
waders a rich food supply. They can easily meet their
daily eiieigy lequiieineiits by switching between prey
species and by roaming over a large area. The winter
remains, however, a difficult period for waders in the
Wadden Sea because of die enhanced costs of living
and the lower intake rate (Ch. 2. 9. 11). That is why
most wader species leave the Wadden Sea in late summer
or autumn to winter further south, but large numbers
of Oystercatchers remain to w inter in ihe Wadden
Sea. Many birds die in severe winters, but even in mild
winters the mortality increases when their intake rate is
low(Ch. 11).
What do we need to know about carrying
capacity?
Has our research contributed to answer the question
about the carrying capacity? First, models developed
in the laboratory to predict the intake rate during feeding
as a function of prey density, appear to be much loo
simple to describe the natural situation. Predators do
not feed in a mechanistic way. because they are versatile
and continuously make decisions. The optimal
prey choice model, based on ihe assumption that
predators maximize their intake rate, appears to be a
helpful tool to predict these decisions.
Second, the aggregation and functional responses
of predators have always been described in terms of
bird density and intake rate, respectively, in relation to
prey density. The implicit assumption is that the harvestable
fraction is invariable, or at least not related lo
prey density. This is a misleading simplification, rarely
mei in nature. We were able to define which prey will
not be encountered by waders and which of the encountered
prey will be ignored due to their low yield.
Due to these constraints, the actual food supply is
much less than the total fcxxl supply. The feeding density
and intake rale of waders are not simply related to
the total fcxxl supply, due to the large variation in the
fraction of prey actually harvestable. Hence the bird
responses are more closely related to the harvestable
food supply. By carrying out measurements on an Oystercatcher
in a rather simple experimental condition,
we were able to predict encounter rate with prey, in­
INTRODUCTION
2'
take rate and prey choice on the basis of prey characteristics.
When extrapolated to the field situation, results
were encouraging and allow predictions of |ieiiods
of shortage and therefore enhanced risks (Ch. 11).
Indeed, winters during which Oystercatchers would
suffer due to a predicted low intake rate, agreed with
periods with an enhanced mortality,
Third, our research was based on the study of individuals.
When we became acquainted with them, we
became more and more aware how different they were.
Individuals feeding on the same spot could lake differ-
Black-headed Gull attempting to surprise a Curlew just alter n
found a RagWOtm, this lime probablv uiihuui nieces. The risk that
a prey is stolen increases « hen waders eat large prey wilh long handling
times. The kleptoparasiies may he congeners or other species,
usually gulls