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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
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prey weight (mg)
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Fig. 13, Msiilus. Density of Mussels belonging In two different cohorts
as a function ot llesh weighl during two years. I lie lines conned
ihe course of the change in density and average pnsy weighl as
long as the cohort existed: based on monthly samples in the Nes
area, from October Of August in the year ol settlement. I*'K4 and
l l 'K5 respectively, onwards. In each winter there was ,i decrease ..I
Ihe prey weight due to a decline in ihe body condilion.
- Macomay-Scrobicularia
-Mya rCockle
latum disappeared in ihe first year of life (Fig. 13) by
which time the survivors had attained 30 to 60 mg
AFDW. It is unlikely thai Oystercatchers will take first
year Mussels. Even second year Mussels provide a
rather low intake rate. Hence, in our study period, the
Mussels did not reach a weight sufficient to make them
a harvestable prey for Oystercatchers ((loss-Custard cl
al. 1996c).
Did prey selection conform to prediction?
The previous sections predicted the highest possible
intake for birds feeding on Cockles I Fig. 8D). Scrobicularia
(Fig. 9D), Macoma (Fig. I0D). Scrobicularia +
Macoma (Fig. 4), Mya (see text) and Mussels (Fig.
13). II birds maximized their intake rate, we would expect
thai they selected the prey species that delivered
the highesi iniake rate. The predicted highest intake
rale varied between 0.2 and 5 mg s ' (Fig. 14). To
achieve this intake rate, the birds had to change their
prey choice as indicated in Fig. 14. Mussels should
never be taken in the study area. Scrobicularia and
Macoma were predicted to be the dominant food resources
in summer, whereas Oystercatchers were expected
to take Mya and Cockles predominantly in winter.
Did the birds behave as predicted in Fig. 14? There
were four periods for which this could be cheeked.
(1) It was predicted thai the birds should take Mya
Fig. 14. Predicted intake rate when ihe fcxxl supply consists of Cockles. Scrobicularia, Macoma, Mya and Mussel-, assuming that the birds
lake the prey providing the highesi iniake rate: based on Figs. 8D. 9D. 10D and 13. lor Mya see text. The predicted prey selection is indicated.
Mussels should never he selected.
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