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

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J F M A M J J A S O N D
FOOD SUPPLY HARVESTABLE BY WADERS
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tributed partly to variation in siphon weight and also
thai ihe difference in depth between winters depends
on the size of the siphon.
Seasonal variation in the accessible fraction of
Macoma and Scrobicularia
Figures 7 to 9 show the average seasonal variation in
the burying depth of Macoma. Scrobicularia and
Nereis. In order to establish how many prey were
really accessible to buds, ii is necessary to measure ihe
proportions that were out of reach of their bills. This is
shown in lig. 10 (the upper 2.4.6 cm. and below 6 cm.
for Macoma) and in Fig. 11 (the upper 4, 6. 8 cm. and
below 8 cm for Scrobicularia).
The dala lor Scrobicularia are only given for ihe
first lour years of sampling, since the species was too
rare during the last 3 years to provide a reliable depth
distribution. Data similar to those shown in Figs. 10
and 11. have already been given for Nereis by Esselink
& Zwarts (1989). As the results show, the seasonal
variation in the accessible fraction varied considerably
between years. The response of the waders will be
considered in several sections of Discussion.
Discussion
Response of waders to fluctuations in their food
supply
The variation in biomass modifies the feeding
conditions for waders in several ways. The greater part
of the seasonal variation in biomass (Fig. 5) may be
attributed to the variation in weight of individual prey
of similar size (Fig. 3; Fig. 8 in Zwarts 1991), to
growth in length and to changes in the prey density
(Beukema 1976. Goss-Custard el al. 1977a). A decrease
in the prey condition could directly affect the
daily consumption rate, unless the birds are able to
compensate by either an extension of the feeding
period and/or an increase of their rate of attacking
Fig. 8. Burying depth of Maconui balihica 115 mm) and
Scrobicularia plana (35 mm) and burrow depth of Nereis
diversicolor (Idem) during the course of the year, split up tor seven
categories ol sci water temperature. The graphs are based on the
same data as in Fig. 7.