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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
1980 1981 1982
Fig. 4. Predicted iniake rale in Oystercatchers feeding on A. Scrobicularia.
B. Macoma compared lo when an optimal mixture of both
prey specie- is taken. The intake rates are calculated on the assumption
that the birds performed optimal depth selections, as shown for
three days in Fig. 3.
Oystercatchers feeding on Macoma would reach the
highest intake rate if they took all prey from the upper
4 cm of the substrate and ignored all prey living more
deeply (Fig. 3B). In contrast, if Oystercatchers restricted
their diet to Scrobicularia. they would have to
probe as deeply as possible and lake all prey within
reach of the bill. In the latter case, the intake rate would
be higher than if only Macoma were taken. Hence, the
prediction was thai the birds would take Scrobicularia.
The intake rate would decrease if the birds added Macoma
to a diet of Scrobicularia. so for this day the prediction
remained that only Scrobicttlaria would be selected.
On 25 November 1982, the highest intake rate
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could be achieved by selecting Macoma living in the
upper 3 cm or Scrobicularia from the upper 5 cm (Fig.
3C). However, the birds would maximize their intake
rate if they selected prey of both species from ihe upper
4 cm, in which case 85% of the biomass would
consist of Macoma,
Calculations such as depicted for these three dav s
(Fig. 3) were repeated for all 88 days of sampling.
There were 53 sampling days out of a total of 88 during
which both prey species were common. In this period.
Oystercatchers rarely raised their intake rate by
taking both species (Fig. 4). They even lowered their
intake rate on seven days by adding Macoma to a diet
of Scrobicularia (Fig. 4A); in contrast, it hardly affected
their intake rate when added Scrobicularia to a
diet of Macoma (Fig. 4B). Since the intake rates predicted
for birds feeding on Macoma or on both species
did not differ much, we decided to treat these two prey
species as a single species for calculations on intake
rate and prey choice.
Mya In principle, a similar depth-related model might
be developed to predict the intake rate for Oystercatchers
feeding on Mya. However, in this prey, the siphon
holes are sometimes visible at the surface, by which
they may be located by sight. This made a model based
on randomly probing the mud less appropriate. Mya
are only harvestable by Oystercatcher during a short
period of their lives, being too small to be profitable
before the second growing season and buried too
deeply to be accessible after the third (Zwarts &
Wanink 1984). Hence, the prey was harvestable by
Oystercalchers during only 2 of the 10 years of observation.
The intake rate was actually measured in one of
these two winter half years.
The birds achieved an intake rate of 1.86 rags in
October 1980 (correcting for the 30% overestiination
of prey weight by Zwarts & Wanink 1984; see Zwarts
et al. 1996c). We know that the birds continued to feed
on Mya in the following months but did not measure
the intake rate. The body condition of the prey decreased
gradually by 20% from November to February,
but the decline in intake rate would have been
larger because the Oystercatchers depleted their food.
The birds eliminated 80% of all the Mya. and 90% of
the shallow, most profitable prey (Fig. 5). Consequently,
the search time must have increased during