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

PREY SIZE SELECTION AND INTAKE RATE
10 16 18 20* io 12 14 16 18 20+
length of Macoma (mm)
Fig. 4. Macoma balihica. The relative risk nl bivalves ol different lengths being laken by Oystercatchers. calculated as the ratio per size class
of relative number of prev taken 10 the relative number in the prey population: data from Fig. 3. A. Relative risk (± SE) when Ihe density of large
Macoma (18 to 22 mm long) was lower than 15 m : (n = 10) or higher than 30 m ; (n = 4); B. Relative risk (± SE) in April to June
(n = 10) and in August to March (n = 6). tortile out the confounding effect of prey density shown in A., a selection is made for studies where
ihe density of Manama 18 to 22 mm long is less than 30 m '.
of ihe substrate (Hulscher 1982). When the frequency
distribution of the size classes actually within teach ol
the bill is calculated (Fig. 3B). (he selection for larger
size classes is even more pronounced because all small
prey, but only a small proportion of the large prey, are
accessible. But when the increase in effective touch
area with si/e (Table 1) is also taken into account, the
probability that small prey will be encountered
dropped to a third of the previous value. Even so. there
still remains a remarkable deviation between the observed
size selection and that expected ol" a randomly
probing Oystercatcher (Fig. 3C).
In all cases studied. Oystercatchers rejected Macoma
less than 11 mm long, while the size classes 11
to 15 mm were taken much less frequentlv than would
be expected on the basis of random searching. The relative
risk of the medium-sized Macoma being taken
appeared to depend on the density of the larger ones.
18 to 22 mm long (Fig. 4A). but not on their own densiiv.
There was also a difference in size selection between
summer and the rest of the year (Fig. 4B); any
confounding influence of the variable density of large
Macoma was removed by restricting the analysis to
studies where the density of large Macoma was less
than 30 specimens m : . In conclusion. Oystercatchers
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concentrate their feeding on the largest Macoma. but
when these are less common, llicv take relatively more
of the medium-sized ones, especially in autumn and
winter. Small prey are always rejected, however.
Scrobicularia plana
In contrast to Macoma. where different year classes
regularly occur together, the frequency distribution of
Scrobicularia si/e classes varies a lot depending on
how many years have passed since the last recruitment
took place. Three studies have been made of si/e selection
in Oystercatchers taking Scrobicularia by
touch. Blomert el al. (1983) studied Oystercatchers
feeding on Scrobicularia between 24 and 48 mm long.
The birds hardly took anv prey larger than 37 mm because
they were inaccessible (see Fig. I). Prey smaller
than 28 mm long were taken less than expected.
whether or not a correction was made for the effective
touch area and the fraction that was accessible. Hughes
(1970a) found that Oystercatchers rejected prey
smaller than 20 mm long when prev 25 Eo 40 mm long
were available. Habekone (1987) showed that small
Scrobicularia were taken when there were few large
specimens. In his study the birds rejected prey smaller
than about 13 mm long and selected the largest size