Mike Hines described the foot-trampling behaviour of Silver gulls on
North
Stradbroke Island. I have noticed this behaviour many times in Silver gulls
during my stay in Australia in 1993 (even on the selfsame island!).
Foot-trampling is a behavioural adaptation that occurs in many gulls.
here
in N.Norway the "best tramplers" are the Common Gulls Larus canus. They use
this a lot on the intertidal sand- and mudflats: they "walk in place",
while slowly moving backward, leaving very characteristic tracks on the
flats (I published some pictures of such tracks in a Norwegian journal, in
case anybody is specially interested).In one area I studied the available
food, small bivalves Macoma baltica and amphipods Onisimus littoralis.
Apparently the gulls preferred the amphipods, as the bivalves had often dug
down again inside the tracks.
What happens apparently is that the trampling liquifies the bottom
sediment, so that the animals "float up". One can easily show this by doing
the same thing oneself.
Black-headed gulls Larus ridibundus also foot-trample, but they do this
in
different environments, i.e. in pastures, where they chase earthworms. The
mechanism here must be a quite different one. Nico Tinbergen postulated
that maybe the tremors caused by the trampling gull confused the earthworm
into "thinking" a mole was near, so that they tried to escape to the
surface; a wonderful story, but as far as I know never proven.
I have seen Silver Gulls foot-trampling in the intertidal many times. Do
they also do it in pastures?
Wim Vader, Tromsoe Museum
9037 Tromsoe, Norway
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