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

INTRODUCTION
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The macro/oohenlhos sampled by a corer with a surface area of 178 cm 2 and 40 cm deep The core was sieved through a I-mm mesh screen.
so heavy a grazing pressure on the siphons in spring
and early summer that the bivalves not only end up
with short siphon, but also a poor body condition (see
pag. 14-15). The prey are thus forced to live near the
surface and expose themselves to a higher risk to be
taken by birds.
Only lean prey may be accessible
Bivalves and worms living close to the surface are in a
poor condition compared to congeners of the same size
living more deeply. Since waders can only feed upon
bivalves that are within reach of the bill, the intake rate
during feeding is overestimated when this is not taken
into account (Ch. 6). The degree of overestimalion can
be calculated when the depth selection is known, as
well as ihe relationship between prey condition and
burying depth.
16
Surface activity of worms
Worms usually live out of reach of the bill, but they are
extremely vulnerable to predation when they come to
the surface to defecate (e.g. Lugworms Arenicola marina)
or to make feeding excursions at the surface (e.g.
Ragworms Nereis divcisicolor). The tidal variation in
the feeding activity of Ragworms is described in chapter
14 and the response of Curlews in chapter 15.
Curlews use two sirategies when they feed on Ragworms.
When the worms live in shallow burrows.
Curlews walk slowly, carefully searching for burrow
entrances, after which the worms are extracted from
ihe burrows, but when the burrow depths exceed the
bill length, by which worms can safely retreat out of
reach of the bird. Curlews walk fast to increase the encounter
rale with worms emerging from their burrows.
In practice. Curlews mix both Strategies depending on