SHOREBIRDS USING THEIR FEET IN PROBING
This is a topic that has long interested me, and I think I may have written
about it to Birding-aus years ago (My home computer has crashed, and I have no
access to my archives just now). Just the other day I watched a Northern
Lapwing foot-shivering on very wet grassland: it stretches out one foot and
very rapidly shivers it. as far as I can see, the foot barely touches the
ground, making the often heard suggestion that this drives worms out of the
ground not a very convincing one, to my eyes. I think that it is more probable
that the movement frightens small prey animals, so that they move and are
easier seen by the lapwing, as all plovers an eye-hunter. I do not know if this
foot-shivering occurs in many Vanellus lapwing species; we don't have more than
one here in N. Norway. Similar foot movements with one foot at the time,
although the shivering is at a much lower frequency, can be seen in different
species of egrets, and I should think the aims are the same, i.e. getting prey
animals to move and become more conspicuous.
Foot-trampling by gulls, which is of common occurrence in many of the smaller
gull species, is quite different. Here the gulls trample with both feet ,
either at one spot, or slowly moving backwards; I have years ago (1975?)
published pictures of the tracks this leaves on mudflats in a Norwegian journal
The Common Gull Larus canus (I am too old to be able to learn the recent
splitting of Larus into a number of smaller genera) is an inveterate trampler
on mudflats, and I've watched the behavior in a number of other species all
around the world: Ring-billed and Bonaparte's Gulls on the US east coast,
Hartlaub's Gulls in S. Africa and also Silver Gulls in Australia. In all these
cases the aim of the behavior is IMO the liquefaction of the substrate mud,
which makes small prey animals 'float up'. One can easily imitate this process
by 'hand-trampling' at the same places.
Also our Black-headed Gulls L. ridibundus trample quite a lot, and they are the
species where the behaviour was first observed. But they also trample on
grassland (Wonderfully watched recently on quite dry grassland in a small park
in Odijk, Netherlands), and it was there that the behaviour was first observed,
i.a. by the great Nico Tinbergen. Also here the 'driving out of worms from the
ground' is one of the theories for explaining this behavior, and in this case
liquefaction of the substrate of course can not be the right explanation. As
far as I can see, the foot-trampling of the gulls on mudflats and on grassland
seems to be exactly the same behavior pattern, so the question is : what came
first? As gulls originally are shore birds, I think maybe the mudflat trampling
is the original one, but I may be wrong.
Nobody has, as yet, mapped in which gull species this foot trampling occurs; I
have not seen it in the large white-headed gull species, but it seems
widespread in the smaller species.
Wim Vader, Tromsø Museum
9037 Tromsø, Norway
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