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

duration of daylight with latitude is probably not
responsible for the variation in densities shown in Fig.
16. as it differs in December by only 70 min. or 12%,
between the latitudes of 50° and 58°.
The final possibility is thai a decrease in the
harvestable food supply in autumn forces waders to
leave the area. For a variety of reasons, the feeding
conditions for waders are worse in winter than in
summer (Goss-Custard et al. 1977a). The question is
whether the feeding conditions are even worse in areas
with lower average surface temperatures. There arcfour
arguments why this may be so.
(1) In summer, waders may choose between about
30 different prey species that occur in the intertidal
zone, including various species of bivalves, worms.
snails, shrimps, crabs and fish. However, the variety of
prey species on offer in winter is restricted Several
epibenthic species, which are very common in summer,
leave the tidal zone to winter in deep water,
including the Common Shrimp Crangon vulgaris
(Beukema 1992a). Shore Crab Carcinus maenas (Beukema
1991). Plaice I'leuronectes platessa (van der
Veer et al. 1990). Flounder Plaiicluhys flesus (van der
Veer et al. 1991) and Common Goby Pomatoschisius
microps (van Beek 1976, Jones & Clare 1977). Waders
that feed on such epifauna, such as the Greenshank
Tringa nebularia (Swennen 1971) and Spotted
Redshank Tringaerythropus (Holthuijzen 1979), have
no other choice than to continue their migration
onwards in late summer and autumn. But as Ihe entire
epifauna vacate the tidal zone during the autumn
everywhere in NW. Europe, this does not explain ihe
differences in wader densities as shown in Fig. 16.
(2) As has been shown in this paper, some prey
species live at a greater depth in winter than in summer.
Hence they are either out of reach, or less profitable
as prey because of the longer handling time.
Furthermore, when only a small proportion of the prey
is accessible, it is likely that the birds must eat
marginal prey with poor body condition (Zwarts &
Wanink 1991).
It is unknown whether the depth distribution in
w inter differs geographically, but the burying depth of
Macoma has heen measured in six different places in
NW. Europe. Unfortunately, the depth measurements
in the Ythan estuary, Scotland (Chambers & Milne
1975a) and in the Danish Wadden Sea (Madsen &
FOOD SUPPLY HARVESTABLE BY WADERS
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Great Britain
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Fig, 17. Seasonal variation in ihe taction of Ihe biomass of
Macoma balihica harvestable by Knot in the Wash (Reading &
McGronv 1978). Humber (Ralcliffe et al. 1981. Evans 1988).
Moreeambe Bav I Evans 1988) and along the Frisian coast (highest
and lowest values found III seven years). Reading & McGrorty
119781. Evans (1988) and Ralcliffe el al. 119811 made slices in the
sediment core and counted the number of Macoma per depth
category. Accessible to Knot are Macoma found in the slices I) to 3
cm. The slice technique gives in fact ihe distance heivveen surl.icc
and a point halfway between the upper and lower edge of the shell,
and not. as in the 'core sampling method' used hv us. the distance
between surface and the upper edge. To make our data comparable
to the English measurements, we calculated the fraction ..I ihc
biomass of pre) found m the upper 2 J cm.
Jensen 1987) were presented with insufficient details
to make the data comparable lo the other studies (Fig.
17). Fig. 17 shows the variation in the fraction of
biomass oi Macoma (9 to 13 mm) living in the upper
2.5 cm. All studies found that 40 lo 100% of these prey
were accessible to Knot in summer, against less than
15*31 in winter. Many Knot leave the Wash in early
autumn and spread out over other British estuaries