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

autumn and spring, and the contrary seems to be the
case. For example, the northeastern part of the Wadden
Sea is visited by hundreds of thousands migrant
waders in .p:-.;;-2 and Scotland, NW. England ami the
German and eastern pan of the Dutch Wadden sea are
relatively important staging areas lor waders in late
summer (Prater 1981. Smit 1984a. Prokosch 1988).
The average surface temperature in February is
1.5 *C in the northern and eastern part of the Wadden
Sea and 7 to 8 °C along die English and French coasts
(Fig. 16). The tidal flats in the Danish. German and
eastern part of the Dutch Wadden Sea tidal flats are
frozen and ice-covered for a period during more than
half of winters, while this occurs only very rarely
further south along the continental coast or in Great-
Britain (Bohnecke & Dielrich 1951). Birds may thus
avoid the northern continental tidal areas to reduce the
mortality risk due to frost. In cold winters, many
waders die of starvation or after they have left the area
in a hard weather exodus I Hulscher 1990. Meininget el
al. 1991).
Wintering in the north is energetically more
expensive due to the lower prevailing temperatures and
the associated higher costs of thermoregulation. A
drop in temperature increases energy expenditure,
especially in small birds (Kersten & Piersma 1987,
Wiersma & Piersma 1994). This may explain why
small waders are most common in the tropics while the
majority of the waders wintering in the north are large
(Smit 1984b, Zwarts et al. 1990a. Meininger et al.
1991. Piersma 1994). Thus, the two largest waders.
Oystercatcher and Curlew, dominate in colder areas.
However, this may also relate to their ability, as in
Redshank (Goss-Custard 1969). to supplement their
tidal flat feeding at low water with extra food from
neighbouring grassland at high water (Heppleston
1971. Daan & Koene 1981. Townsend 1981). h is
striking thai during the winter these three species in the
Dutch part of the Wadden Sea are most commonly on
lidal flats lying adjacenl io extensive inland grasslands
and occur in only low numbers where such areas are
not available (Zegers & Kwint 1992).
A third reason lor not staying in northern intertidal
areas might be that the day length is so short in winter
lhat birds that cannot feed, or can only do so less
successfully, at night face a considerable reduction in
their feeding opportunities However, variation in the
FOOD SUPPLY HARVESTABLE BY WADERS
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