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

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PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
0 loss ol body weighl
| mass mortality Coddes
• mortally (unknown reason)
| predation Oystercatcher
ii?
S?:A
70 80 90 100 110 120
biomass at 15 August (guv 2 )
Ki 1 *. 18. Loss of biomass by Cockles. Scrobicularia. Macoma. Mya
and Mussels between 15 August and 15 March of the next year in rehiiion
to the total biomass present at 15 August. The loss of biomass
is due to a decrease in Ihe body condilion and to mortality, the latter
given separately for Oysiercatchcr predation, mass mortality in
Cockles (see text) and other causes. Panel A. gives the absolute loss
of biomass (g m : ). and B. the loss as percentage.
when the food supply was poor and this increased to
17% in winters with a rich food supply.
Discussion
Model assumptions
In this paper we predicted the feeding behaviour of the
Oystercatcher on the basis of measurements on the
prey. This required several simplifying assumptions.
Before we discuss our results in some detail, it seems
prudent to investigate some limitations in our approach.
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Spatial variation In our model calculations we ignored
the spatial variation in prey density. The samples
of the macrozoobenthos in the Nes area were taken at
73 sites and we know from this mat Macoma and Scrobicularia
on the rich sites were twice as common as on
the poor sites and that the ratio was even three in Cockles
and Mya (Zwarts 1988b). In principle, it would
have been possible to repeat all calculations done for
the Nes area as a whole for the 73 sites individually.
This would not have changed the average trends shown
for prey choice and intake rate. However, whereas we
predicted that all birds would everywhere have
switched at the same moment from one prey to the
other. Ihe timing would have been different between
sites, so that more gradual changes would be expected
for the Nes area as a whole.
Feeding specializations First, we assumed that all
birds selected one prey species, with the exception that
they could take a mixture of Scrobicularia and Macoma.
This was certainly not the only exception, because
birds also took Cockles and Macoma. This was
only observed, however, in summer when Macoma
lived close to the surface at the same depth as Cockles
(Hulscher 1976. Hulsman unpubl.). Consequently, we
were probably wrong in predicting thai birds would
have taken only Macoma in summer 1984 and 1985
and ignored Cockles.
Another simplification was that all birds should
perform the same prey selection, whereas direct observations
also showed that this was not true. For instance,
one female took only Ragworms among the 29
other colour-banded birds feeding in the Nes area on
Scrobicularia and Macoma (Blomert et al. 1983). Bill
length has a large effect on the prey selection. The last
birds feeding on Scwbicularia in October 1979 and
the first ones in early spring 1980 were all females
with long bills, because the prey then lived out of reach
of the shorter bill of the males. Also the depletion of
the second year Mya in autumn 1980 was due to predation
by females. The bill length of the birds feeding
on Mya was. on average 1 cm longer than for the birds
feeding at the same time on a cockle bed, just north of
the study area. Hence, it is to be expected that the seasonal
variation in intake rate and harvestable food supply,
of which the averages has been given in this paper,
were larger for males and smaller for females.