FOOT-MOVEMENTS IN FORAGING BIRDS
Recently there has been a flurry of observations on Birding-Aus on rapid
foot movements in plovers, with Bassian thrush, silver gulls and herons
being involved by some contributors. This is a most interesting subject,
and in my opinion more complicated than appears from these messages, in
that several different types of movements should be recognized. I guess
that most of these are innate, but that the birds will have to learn by
experience when and where best to use them. I remember that Heinroth`s
hand-reared gull-chickens practiced foot-pattering for long periods, on a
wet floor-cloth, and started up "automatically" as soon as placed on this
underground.
What gulls do,-- and as mentioned in an earlier message on the
subject, I
have watched Silver Gulls practice this often--, is foot-pattering
(a.k.a.foot-trampling), where they "walk in place" or slowly backwards, all
the time intently looking between their feet, and picking up small
organisms that come to the surface because of the liquefaction of the
bottom-sediments.The activity leaves very characteristic tracks in the
intertidal and I have published photographs of such tracks from N.Norway.
(One can easily mimic this by "hand-pattering" and see that it works.
Here in N.Norway the champion-patterer is the Common Gull Larus
canus,
while I never have seen any of the large gulls patter in the intertidal.
Black-headed Gulls L.ridibundus (A much closer relative of the Silver Gull)
also patter, but they do this much more often on meadow-land (and there
Nico Tinbergen has described the action also for Herring gulls
L.argentatus). The classical explanation for that is that the pattering
"scares up the earth-worms, who "believe" there is a predator below
ground"; personally I wonder if this is not an oft-repeated myth, without
any experimental or observational evidence behind it at all, and just
copied from one paper to the next.
Together with the Black-headed Gulls in the meadows there are
often
Northern Lapwings Vanellus vanellus, and they have a similar but different
foot-movement, i.e. they rapidly move one foot in a trembling movement on
the surface. This is the same movement, possibly, that several
Birding-aussers have noticed in other plovers and dotterels. I have always
thought, but again without any evidence at all, that this movement served
to make eventual prey animals move and therefore become more easily
visible----I have the vague memory that the lapwing foot movements have
been discussed by W.Barbard in his wonderful book "Gulls and Plovers", but
I do not have that available here. (I also have seen similar foot-trembling
described from Bassian Thrushes in a short note somewhere, but again can
not remember where or when.)
When on a visit to India 10 years ago, I watched White-tailed
Lapwings
Vanellus leucurus at Bharatpur, and wrote in my notes: "These birds
regularly used "leg-trembling"--with one leg, and through the water rather
than through the mud, it looked like--, the only lapwing here that I did
see carry out foot-trembling".White-tailed Lapwings, with their very long
legs, are among the "wettest" of all lapwings.
This seems already much closer to the various forms of
foot-trembling and
"leg-stirring" that one can observe in herons and egrets, and that have
been described in the literature.
The study of such fixed foraging patterns is very fascinating,
just as
e.g. observations of the different forms of bottom-scratching practiced by
different sparrows, thrushes, gallinaceous birds, lyre-birds and
scrub-fowl. It is a good example of convergence: the function is usually ca
the same, but the aims have been reached starting from different muscle
movements and -patterns.
Wim Vader, Tromsø Museum
9037 Tromsø, Norway
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