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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
classes were selected: (1) Herring Gulls took Mussels
in 1985 that were still too small to be taken by Oystercatchers
(Zwarts unpubl.): (2) Knot Calidris canutus
selected medium-sized Macoma hardly taken by Oystercatchers
(Zwarts & Blomen 1992): (3) Oystercalchers
look second year Mya still ignored by Curlews
(Zwarts & Wanink 1984). Hence, although different
bird predators successively coniribule lo the decline ol
the prey cohorts, there was no reason to take into account
the predation pressure of the other bird species
when we compared oystercatcher predation to the
elimination of the prey biomass harvestable by Oystercatchers,
Seasonal variation in intake rate and prey selection
The intake rate of Oystercalchers varied seasonally,
being high in summer and low in winter. This trend
was more pronounced in the burying prey species,
Scrobicularia and Macoma, than in the surface prey,
the Cockle (Fig. ISA). The seasonal variation in intake
rate was even smaller in Mussel-eating Oystercatchers
(Goss-Custard & Durell 1987: Fig. 17 in Zwarts et al.
1996b). The explanation is that, due to the variation in
burying depth, the encounter rate with burying prey
was reduced in winter. There was no such a difference
in surface prey, although birds stabbing the bill betvveen
Ihe valves may more often encounter closed bivalves
in winter than in Ihe summer, when Cockles and
Mussels (ccd more often themselves. This may explain
why the seasonal variation in intake rate in Mussel-eating
Oystercatchers was larger in slabbers than in hammerers
(Goss-Custard & Durell 1987).
According to the predictions (Fig. I5B) and direct
nbservaiions (see text), burying prey were selected in
summer and only laken in winter when there were no
surface prey. Several other studies provide similar evidence
for such a seasonal shift in the diet of the Oystercatcher.
Bunskoeke et al. (1996) and Hulscher ct al,
(1996) show that Macoma was the main prey on
Schiermonnikoog in spring and completely disappeared
from the Oystercatchers diet in late summer.
That Macoma is indeed a summer prey, hardly taken
by Oystercatchers in winter, is also evident from the
work of Beukema (1993a) on the Balgzand in the
western part of the Wadden Sea. He found that the
monthly mortality of Macoma between mid-March
and mid-August (five months) was. on average, three
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limes as large as in the remaining seven winter months
(Beukema 1993a: Fig. 6). Bird counts indicated thai
ihe monthly predation pressure by Oystercatchers was
only 1/4 the level in the five summer months compared
to the seven winter months (Beukema 1993a: Fig. 4).
Assuming that the mortality of adult Macoma was
completely due to oysieicalchei predation. ii follows
J F M A M J J A S O N D
Kig. 19. Seasonal variation in ihe number of