foot quivering in thrushes

Back in 1998 (time flies!) I wrote a little piece on these lists on foot paddling in gulls: I have enclosed a copy here, as few subscribers will be the same after 20 years.

 

FOOT-MOVEMENTS IN FORAGING BIRDS (Copied from 1998)

 

                              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 or foot-paddling), 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 (In Norwegian). 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. Barnard 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 cannot 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

 

A few years ago I could observe very nicely ‘foot stirring’ in a Little Egret in  Kyoto, Japan, where the herons and egrets were very tame and often accompanied fishermen along the river. This leg stirring has been described for many small egrets both in America and Australia; especially the Snowy Egret has very often been observed to practice leg stirring. Most probably the  objective is also here to get potential prey animals to move, so that they become easier visible . As far as I have been able to establish, this method of foraging has not been observed in the large Ardea herons, i.a. the Great Egret, but there are observations of Reef Herons leg stirring.

 

Two years ago, I participated in a birding tour to Sri Lanka, and one of the new birds I saw there is the Sri Lanka Thrush Zoothera imbricata; we watched these birds in Sinharaja forest and I was elated to see regular and repeated Food quivering, i.e. the rapid quivering of one foot, usually just above the ground among the fallen leaves. This foraging method has long been known among Catharus thrushes, especially the Hermit Thrush. At first there were various explanations for the behaviour, such as  ‘moderate threat behaviour,’ or  ‘displacement activities’ , but more recently most authors seem to agree, that this is indeed a foraging method, aimed at getting small prey-animals to move and thereby become more visible. The behaviour has been noted for a number of Catharus thrushes in the US, and, as I already noted back in 1998, apparently also in the Bassian Thrush in Australia, a close relative of the Sri Lanka Thrush. (I still have not found the relevant reference). Very similar behaviour has also been noted and studied in the New Zealand Robin Petroica australis (see Berggren 2006), a member of a quite different family.

 

I shall be very grateful for further observations of these various food movements in birds.

 

Wim Vader, Tromsø, Norway

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