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

FOOD SUPPLY HARVESTABLE BY WADERS
HOW THE FOOD SUPPLY HARVESTABLE BY WADERS IN DE
WADDEN SEA DEPENDS ON THE VARIATION IN ENERGY DENSITY,
BODY WEIGHT, BIOMASS, Bl IRYING DEPTH AND BEHAVIOUR OF
TIDAL-FLAT INVERTEBRATES
For several reasons, waders in the Wadden Sea face a large seasonal and annual variation in their food
supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that:
- (I) The average energy density of ten invertebrate prey species varies between 21 and 23 kJ g '
AFDW. In Scrobicularia plana and Mya arenaiia. but not Macoma balibica. the energy density is 10%
lower in winter than in summer.
- (2) Depending on the species, the body weights of prey of similar size are 30 to 60% lower in winter
than in summer.
- (3) The year-to-year fluctuation in standing crop biomass is larger in some species than in others, the
difference depending mainly on the frequency of successful recruitment. The overall biomass of the
macrobenthos in winter is half of that in summer, but the timing of the peak biomass differs per species.
- (4) The burying depth varies per species: Cerastoderma edule live just beneath the surface, while
Macoma. Scrobicularia. Mya. Areiiicola marina and Nereis diversicolor bury more deeply and the
majority of these prey live out of reach of the bird's bill. In all six species, burying depth increases with
size. There is no seasonal variation in depth of Cerastoderma and Mya, but the four other species live at
most shallow depth in early summer and most deeply in mid winter. Burying depths in winter vary from
year to year, but are unrelated to temperature. Neither has temperature any effect on depth within
months. For Knot Calidris caiiutits feeding on Macoma. the fluctuation in the accessible fraction was the
main source of variation in the biomass of prey mat is actually harvestable, i.e. the biomass of prey of
suitable size that is accessible.
Accordingly, the paper reviews the available data on the temporal variations in accessibility.
detectability. ingestibility, digestibility and profitability of prey for waders. Only a small part of the prey
is harvestable since many accessible prey are ignored because of their low profitability, while many
profitable prey are inaccessible. The profitability of prey depends on their size and weight but also on
their depth in the mud. since handling time increases with burying depth. A simple biomechanical rule
explains why the handling time of small prey increases with bill length and why large, long-billed
waders ignore a disproportionately larger part of the small prey. The fraction detectable for visually
feeding waders is usually very low. especially when the temperature of the substrate is below 3 to 6 °C.
Waders vary their prey choice over the year in response to the changes in the availability and profitability
of their different prey species. The food supply harvestable by waders is much lower in winter than in
summer. For waders wintering in the Wadden Sea. the food supply may be characterized as
unpredictable and usually meagre. Waders wintering in NW. Europe are concentrated in coastal sites
where the average surface temperature is above 3 °C. This probably cannot be explained by a greater
burying depth, and only partly by a lower body condition, of prey in colder areas. Yet the harvestable
fraction is lower in colder sites, especially for sight-feeding waders, as invertebrates arc less active at low
temperatures. However, the lower energetic cost of living and reduced chances of the prey being covered
by ice may also contribute to the waders' preference for warmer sites.
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