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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
Fig. 17. Flimination of biomass for A.-D. four different bivalve
species (g m : month) and R. calculated consumption
(g m : month ')by Oystercatcher assuming lhat all birds lor
aged each month onlv on the prev species thai delivered the
highest intake rale (Fig. 14). The elimination, summed for
different cohorts, has been define hid of average
prey weighl and the numbers thai disappeared in -' month'
The apparent negative elimination in Macoma is due to resettlement
by which the density rose from March to May.
The elimination of the small size classes, which would have
been ignored by Oysteicatchers. and the large Mya. lhat
would have been out of reach of ihe Oysicreatcher's bill, are
marked separately The total bird consuinpiion is ihe product
of Oystercatcher density (Fig. 6A) and consumption per bird
(varying between 36 g day ' in summer, and depending on
ambient temperature, increasing to SO g on cold winter days).
3 Oystercatchers ha ' in die three years when
Scrobicularia was ihe dominani fcxxl supply and
more than 25 birds ha ' in four years when the
prey biomass of the Cockles reached high values.
The predicted intake rate was clearly below the
critical lower acceptance level of 1 mg s ' in the
four poor winters in succession, 1979-1982.
Comparing elimination of biomass and predation
pressure
The average total predation pressure by Oystercatchers
was estimated at 12 g nr 2 year 1 , but due
to the very large variation in density (Fig. 6A),
the predation pressure varied considerably between
and within years. Assuming that the birds
only took the prey which would have given the
highest intake rate (Fig. 14), the predation pressure
on the different prey species could be compared
to the total biomass that was actually eliminated
(Fig. 17). Note that me production by
elimination of the small prey was usually very
low. except for first year Cockles in 1985 and
1986, and Macoma and Mya in the first year after
the spatfall 1979.
Cockles The prediction was that Oystercatchers
would have fed on Cockles in 1978 and in the
winters of 1984. 1985 and 1986 (Fig. 17E). Indeed,
these were periods of high losses of cockle
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