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

Curlew when ihey both feed on clams, because of the
small overlap between the prev sizes which are available
as well as profitable for both bird species (Fig. 3).
Since Curlews select larger elams than Oystercatchers,
there is a segregation in lime if the two species are to
feed on clams Irom the same year of spatlall.
However, this potential partitioning of resources
does not tell us anything quantitatively about competition
for food between the species. Oystercalchers
might deplete the clam stock completely before the
shells have reached the size at which diey can be harvested
profitably by Curlews.
Depletion of the food stock
Since we started our sampling programme of the maerobenihie
fauna in 1977. there has been one successful
spatfall of clam (1979): the previous one was in 1976.
Spatfall occurs during the summer. First-winter
clams reach a size of c. 8 mm and are thus still too
small to be utilized by Oystercatchers. During the second
growing season most animals pass the lower acceptance
threshold for Oystercatchers. but not until the
next year do they become profitable for Curlews (Fig.
5A). During the growth of the shell, clams bury deeper.
The size-depth relationship (Fig. 3), can be used to derive
the average depth of each cohort (Fig. 5B), but
also allows calculation for all sampling date of the
numbers of clams above the lower acceptance size
threshold and which are accessible lo both bird species
(big. 5C). Good years for clam-eating Oystercalchers
(the winters of 1977-78 and 1980-81) precede the rich
years forCurlews i ihe winters of 1978-79 and 1981-82).
The predation pressure by Oystercatchers and
Curlews appears to be high enough to explain the
greater part of the loss of clams after the second growing
season.
In October and November 1980 all Oystercatchers
present on the mudflats preyed upon clams. From the
work of Hulscher we know that the average intake was
3.63 clams/min (n - 870 min). Because the waler level
was measured continuously, we know that the mudllats
were exposed for 15 900 min in daylight during
these two months and for 21 500 min at night. During
daytime low-water periods. Oystercalchers fed for
88% of the time that the Hats were exposed, at a den­
PREY DEPLETION BY OYSTERCATCHER AND CURLEW
342
sity of 1.26 birds/ha. Combining these data, we calculate
that in two months the Oystercatchers took 6.4
clams/m 2 by day. Bird counts were made also al night.
with infra-red binoculars, and from this it is known
lhat Oystercatchers remained to feed at night, hut their
feeding rate could not be measured. Assuming ihe
same feeding rale as hv dav. the total predation in Octoberand
November amounted to 15.1 clams/m-.
If the birds which were present during the rest of
the winter continued to feed on clams and did not
switch to the only alternative prey available. Macoma
balihica. the resultant predation pressure in the period
1 October 1980 to I March 1981 could be estimated at
20 clams/m-* by day only, or 49 clams/nr in total if the
predation rate by night was the same as by day. This
value is a minimum, since we have omitted one bird
count which took place just after a cold spell, during
which the mudflats were frozen and many Oystercalchers
fed on dying bivalves. During this period the
density rose to 30 birds/ha. 20 times as high as ihe average
density that winter. This situation lasted for between
3 and 5 days. In that short period as many as 20
clams/m 3 might have been eaten if the birds had
achieved the same feeding rale as in autumn.
Between 1 October 1980 and 1 March 1981 there
was a decrease from 110 lo 20 clams/m 2 (Fig. 5C). At
maximum. Oystercatchers look 69 of the 90 clams/m-'
which disappeared. A smaller proportion were eaten
by Common Gulls Lams carats, present after the cold
spell, and by some of the Curlews which started to
swallow the small elams.
No detailed observations are available for the winlet
of 1977-78 when ihe density of preferred size
classes was much higher but also decreased dramatically
during the winter (Fig. 5C). Counts of birds on
the feeding area are available, however, and we also
know that clams were the main (and perhaps the only)
prey taken because we found many clams recently
opened by Oystercatchers. The average Oystercatcher
density in the winter of 1977-78 was 5.1 times as high
as in the winter of" 1980-81. Assuming the same intake
rate by the birds, the predation pressure in the period
October-March would have been 351 clams/m
suming equally heavy predation by day and by night.
The decrease which was found -from 440 to 120
clams/m 2 - was in fact below the estimated impact of
the Oystercatchers.