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

1000 r
1 II" 1 -
r^Jan'80
PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
0.5 1 2 3mg-s-'
10 100 1000
prey weight (mg)
244
Fig. 7. Cerastoderma. Ploi of density in in -i againsi avenge llesh
weighl (mg AFDW) for live different cohorts during one t.i three
yean. The hues connect ihe course of the change in density and average
prey weight for as long as the eolion existed, besed on
monthly samples in ihe Nes area, from August in the year ofsetiement
onwards. In each winter (here was a decrease ol the prey
weight. Grey fields indicate the predicted intake rate of Oy-tei
caichers as ihe combined function of pa*y weighl and prey densit]
(based on Fig. 16 in Zwarts t/u/. I996h).
was assumed lhat all Oystercalchers fed on the prey
predicted to yield the highest intake rate. Except for
Macoma and Scrobicularia. we assumed that the birds
could prey on only one prey species at a time. As
shown before, the actual choice between these two
species did not matter much for the predicted intake
rate, so that it was hard to reliably predict the optimal
mix. Fortunately, Scrobicularia was absent during the
last four years. For the first four years, il was assumed
that both prey were taken in an equal amount of
biomass
Results
Bird density
The Oystercatcher was the most common shorebird in
the study area with 8.4 birds ha ', averaged over ihe entire
year, but there was a large variation in the density
during 8'/, years of counting (Fig. 6A). Hardly any
Oystercatcher fed in the area in 1981 and 1982.
Nonetheless, the peak numbers were present each year
in mid-winter and the lowest numbers occurred between
mid-March and mid-August. No counts were
made when ice covered the mudflats (Fig. 6B), but
usually no Oystercatchers fed in the study area during
such periods, since mosl birds left the area altogether
and those remaining stayed on the roosts. If birds led
on the mudflats during frost periods, they did so outside
the study area on the water's edge near the lowwater
mark, the only place where the substrate was not
frozen.
Oystercatchers and Cockles
The intake rate of Oystercatchers depended closely on
the weight of the prey taken and. to a lesser degree, on
ihe densit) in which these prey occurred. Figure 7 de-