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

When hcnlhic bivalves extend llien siphon
above the surface, the] often lose the mpot the
siphon due to siphon ripping Common
Shdmps and juvenile Shore Crabs.
Does siphon nipping also affect the predation risk
of Mamma' The exclusion of the siphon nippers
caused an increase of the burying depth; in some cases
Macoma even reached ihe w inter depth of 5 cm. The
main predator of large Macoma is the Oystercatcher.
The bill of this species is 7 to 8 cm long and since the
bird may thrust its hill entirely into the mud. one mav
ask w tether Ihe observed small increase of the bun. ing
depth is sufficient lo reduce the risk to be taken by
()\ Mercaichcrs Detailed observations on the depth selection
ot Ovstercaichcrs feeding on Macoma are still
lacking. However, there is enough indirect evidence
that Oystercatchers igaoteMacoma living at 4 to6cm,
because these prey, although siill accessible, are un­
INTRODUCTION
15
profitable, due to the increase of handling time with
burying depth. Therefore. Mamma are alreadv safe
limn bird predation when they live 4 or 5 cm beneath
the surface of the mud. I or the same reason. Macoma
are only a summer prej for Owercatchers. because
their increased winter depth makes iheni too unattrachve
io feed upon. Hence, the subtle reduction ol the
burying depth due to siphon grazing mav increase its
value as pre) and. lluis. the risk lo be predated Concluding,
siphon nipping bj fish, shrimps and crabs facilitates
bird predation. or as Rudi Drent said immediate!;,
when I first showed him the graph: "Schol helpt
Scholekster" (fish helps bird).