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

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
1977 78 79 80 81 82 83 84 85 86
year
Nereis
I Arenicola
B Mytilus
I I Scrobicularia
• Mya
1 Macoma
• Cerastoderma
Fig. 16. Variation in A. total biomass, B. harvestable biomass. C.
the average bird density and predicted intake rate in December. Har-
vcsiahle biomass is defined as the summed biomass. excluding
Cockles < 10 mm long. Scrobicularia < 13 mm long or living > 6 cm
deep. Macoma < 11 mm long or h\ ing > 4 cm deep, Mya < 17 mm
or lit ing > 6 cm deep, and Mussels < 25 mm. Original data are given
in Figs. 8A. 9 A, I0A and 12 for biomass values. Fig. 14 for pre­
dicted intake rates and Fig. 6A for bird densiiies: no bird counts are
available for December I "85 and 1986
252
Seasonal variation in intake rate and prey
selection
In all prey species there was a seasonal variation in the
predicted intake rate, particularly in birds feeding on
Scrobicularia (Fig. 9D) or Macoma (Fig. I0D). The
iniake rate of cockle-eating Oystercalchers was also
predicted lo be higher in summer than in winter (Fig,
8Dl. but the seasonal variation was not as large as in
the two deep-living bivalve species I Fig. 15 A). The explanation
for this difference was that the seasonal variation
in burying depth of the deep-living bivalves
made them very unattractive to feed on in winter, because
the majority of prey were inaccessible to the
probing bird, and if the prey were accessible, they were
hardly profitable (Zwarts el al. 1996b). In contrast.
Cockles remained living at. or just beneath, the surface
for Ihe entire year, so thai the accessible fraciion did
not vary seasonally.
Since the seasonal amplitude in iniake rate differed
so much between the prev species. Oystercatchers
achieved the highesi intake rate when they took the
buried prey. Scrobicularia and Macoma. in summer
and Cockles in winter (Fig. 15B). In this figure we
lumped Scrobicularia and Macoma because, as explained
before, we could not predict accurately enough
which of the two species should be taken.
Annual variation in bird density, intake rate, prey
selection and food supply
Although the number of Oystercatchers present in the
study area varied seasonally, a close look at Fig. 6 reveals
that, in some years, the monthly numbers were
systematically lower than in other years. To analyse
whether these year-to-year variations were due to variation
in the food supply, we examined the data from
December. There was a gradual decrease in the total
winter biomass of the prey species combined over the
years, but the annual fluctuations were large, varying
between 40 and 100 g m 2 during ten years (Fig. 16A).
After subtraction of the biomass of prey either too
small or too deep to be taken, the biomass harvestable
for Oystercatchers appeared to differ even more, being
extremely low in 1979-1982 (Fig. I6B). In the first
three of these years. Scrobicularia contributed about
3/4 to the total biomass, but this part of the food supply
was largely nol accessible to Oystercatchers. The variation
in wader density was even larger, being less than