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 wim.vader@uit.no ===============================
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