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

Introduction
This paper describes the seasonal and annual variation
m ihe food supply of birds foraging on the intertidal
tlais in the Wadden Sea. A quantitative analysis of
actual food supplies demands not only a description of
the fluctuations in energy density, body condition and
total biomass of the several potential prey species
present, but also an investigation of the variations in
the fraction of the pre\ that is available as well as profitable
to the birds. Large prey are usually profitable
(i.e. energetically worth consuming) but not available
(i.e. detectable, accessible and ingestible). On the other
hand, small prey are often readily available but are
[ejected because of their low profitability. Prey that arc
harvestable. defined as profitable as well as available.
Often comprise only a small fraction of the total
biomass of ptev present. In Knot Calidris caniiiiis. a
wading bird specializing on eating hard-shelled prey.
89% of the prey biomass consisted of animals that
were too large or loo small or lived too deeply lo be
taken (Zwarts & Blomert 1992, Zwarts etal. 1992).
Fluctuations in the harvestable fcxxl supply have in
be measured precisely before answers can be given to
questions such as: why do diet and intake rate in
waders vary seasonally? why do waders leave the
Wadden Sea to winter further south? Indeed, this kind
of information is essential for any study of the
relationship between predators and their lood supply.
Il may also help us to explain how predators coexist.
since it clarifies the degree to which there is overlap in
the harvestable fcxxl supply exploited by different
species (Zwarts & Wanink 1984).
Several aspects of the seasonal and annual variation
in the fcxxl supply of shorebirds have already been
described. Unique series of measurements are available
on the year-to-year variation in the biomass of
benthic prey in the Wadden Sea (Beukema et al. 1993).
There is little seasonal variation in the energy densit)
per g AFDW of tidal invertebrates (Beukema & de
Bruin 1977. Chambers & Milne 1979). Beukema
(1974) showed that in winter the biomass of all macrobenlhic
animals combined is about half of that in
summer. This difference is mainly due to a decrease in
the flesh content of individual prey (Hancock &
Franklin 1972. Beukema & de Bruin 1977. Chambers
& Milne 1979. Zwarts 1991). Seasonal variation in the
FOOD SUPPLY HARVESTABLE BY WADERS
48
JFMAMJJASOND JFMAMJ J A SOND
Ki|i. I. Seasonal varialion in the energv densit) ik.l g ' AFDW ±
SE) ul Scrobicularia plana and Mya iircnuriti. Results nf one-way
analyses ol variance are given.
burying depth of benthic animals has been studied by
Reading & McGrorty (1978). Zwarts & Wanink
(1989) and Zwarts & Esselink (1989).
This paper presents additional data from the
Wadden Sea on seasonal and annual variations in (I)
the energy density. (2) llesh weight of prey of constant
si/e. (3) total biomass. and (4) burying depth of shorebird
prey. Taken together, these components comprise
the main sources ol variation in the harvestable food
supply of the birds. We conclude that (1) the variation
in the accessible fraction may be larger than the
fluctuation in the total biomass of the prey actually
present, (2) the extent of the seasonal variation in the
flesh weight and in the accessible fraction differs
greatly between prey species. (3) the food supply
harvestable by waders is much lower in winter than in
summer, and, therefore the most-studied shorebird.
Oystercatcher Haematopus ostralegus, achieves a
higher intake rate in summer than in winter. (4) low
mud temperature in winter reduces the detectable prey
fraction, but probably has no effect on the burying
depth and body condition of the prey, and (5) large
waders ignore a disproportionately large portion of the
smaller, unprofitable prey.