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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
accessible lo Ovstercaichers; see Zwarts & Wanink
(1993) for a detailed description of the seasonal and
annual variation in biomass and prey accessibility.
Human impact There was no human impact on the
food supply in the study area. There was no dredging
for Cockles within the study period and the few people
digging for Lugworms Arenicola marina did not do so
within the Nes area.
Counting and observing the birds
The Oystercatchers. and other bird species, were
counted PA ice a month at low tide from the top of the
sea wall which offered a splendid view over the whole
area: in toial 166 fortnightly counts were made be-
400 600
prey weighl (mg)
800 1000
Pig. 2. A. Average profiiahiliiy (uiy s' handling) and B. avenge iniake
i' u xting • is • function ol t erage " Bigl
laken by Oystei catchers feeding 00 live (Efferent prey species The
profitabilities and intake i.iles are laken from Zwarts el al. (1996b).
The minimal intake rate required to meet the energy rci|inremenl. .il
thermoneuiral conditions i^ indicated lor ihe usual daily range in ihe
available feeding time.
238
tween summer 1977 and autumn 1985. The birds were
dispersed over the feeding area and were counted one
by one. Counts of the birds feeding in the 73 0.1 ha
plois in the Nes area vv ere also often made from ihe observation
tower, but since only a lew nl these counts
were available for the winter months, the series trf
counts made from ihe sea wall will be used instead.
For two reasons we took the bird counts from the eastern
100 ha (Fig. 1). and not from the entire area, as
measure of the bird density. First, as already indicated.
our measure of the food supply in the Nes area was
more representative for the eastern part than for the entire
area. Second, prey and si/e seleciion and intake
rale were Studied in Oystercatchers feeding around ihe
towers in the eastern pan of ihe siudy area. These data
were collected during the first five years of observations
(1977-1981) by Hulscher (1982 & unpubl.).
Blomert el al. (1983). Hulsman (unpubl.), Zwarts (unpubl.)
and Zwarts & Wanink (1984); see /.warts ct al.
(1996b) for a summary. Onlv qualitative data on the
prey seleciion arc available during the latter five years
(1982-1986).
Comparison of [lie low water counts in the 100 ha
and the 7.3 ha around the Nes hide in the centre of this
area showed thai the Oystercatcher density in the Nes
area as a whole was highly correlated (r = 0.93, n = 35)
with the density measured on the same day in the 100
ha being, on average. 1.3 times higher. This difference
was to be expected since the upper 1/4 of the 100 ha.
situated along the dike, was hardly used when the tide
was out. All bird numbers were therefore expressed as
bird densities for the lower 3/4 of the study area, i.e.
situated between 10 cm above and 20 cm below mean
sea level. Despite the high correlation between bird
densities on 100 and 7.3 ha. there was one period
where the counts of 100 ha would highly overestimate
Ihe density on the 7.3 ha. In November 1979. mosl
birds left Ihe Nes area for some months but remained
to feed on the lower shore within the 100 ha. Therefore,
we used the bird densities measured in the Nes
area for these months.
Estimating parameters of the prey choice model
For each prey species, we need to know the profitability
of the various prey classes and their respective encounter
rates. For profitability we can draw on a receni
review (Zwarts et al. 1996b). The profitability is de-