Astute birdwatchers might have noticed that some bird species have comb-like serrations running along one edge of a toe claw. This feature, termed the pectinate claw, has long been assumed to be used just like combs we use for our own hair — to keep clean and look good.
Upon seeing the Magnificent Frigatebird (Fregata magnificens), John James Audubon (1785 – 1851) remarked:
I have frequently observed the Frigate Bird scratch its head with its feet while on the wing; and this happening one day, when the bird fell through the air, as it is accustomed to do at such times, until it came within shot, I killed it when almost over my head, and immediately picked it up. I had been for years anxious to know what might be the use of the pectinated claws of birds; and on examining both its feet with a glass, I found the racks [sic] crammed with insects, such as occur on the bird’s head, and especially around the ears. . . . I now therefore feel convinced, that, however useful this instrument may be on other occasions, it is certainly employed in cleansing parts of the skin of birds which cannot be reached by the bill (Audubon 1835).
The function seems as self-evident to Audubon as it is to us — the pectinate claw combs out all those nasty ectoparasites (i.e., external parasites), especially the feather lice that eat precious down feathers. Most birds have a little overhang at the tip of the beak for picking out and shearing apart lice, but a special foot comb would be ideal for reaching the head and neck areas inaccessible to the beak during preening (Clayton et al. 2010; Bush et al. 2012). However, this tidy hypothesis quickly runs into some problems.
The first is that pectinate claws occur only sporadically throughout the avian phylogenetic tree. A review of 118 bird families found that only 17 possessed pectinate claws. This was a diverse assemblage that included herons, nightjars, owls, frigatebirds, terns, grebes, and cormorants. Curiously, only one family of passerines, the dippers (Clincidae), were observed to have pectinate claws; passerines constitute the bulk of species diversity amongst birds. Even so, within each family, only a handful of species might have this feature. And within certain species, the appearance of the pectinate claw was variable among individuals (Clayton et al. 2010).
So if the pectinate claw served such a vital function as stripping the feathers of harmful parasites, we should expect to find them consistently across a wide swath of bird diversity. Instead, the pectinate claw seems to have evolved independently numerous times at very spotty intervals.
Another source of doubt cast upon the role of pectinate claws in removing ectoparasites comes from a study of Barn Owls (Tyto alba pratincola). While owls with the most number of teeth on their pectinate claws were categorically the least likely to have lice infestations, there was no correlation between the number of lice and the number of teeth on the claw (Bush et al. 2012). Foot claws are undoubtedly a critical tool in keeping a bird groomed, but given the rather ambiguous conclusions reached by researchers involved in these correlative studies, without experimental manipulation — e.g., filing off the teeth of the pectinate claw and comparing parasite loads between individuals — there is little convincing evidence that pectinate claws function specifically to comb out ectoparasites (Clayton et al. 2010; Bush et al. 2012).
As a coda, there is a fascinating tangent related to Audubon’s particular interest in the feet of the Magnificent Frigatebird. His attention was drawn to the presence of the pectinate claw, a trait he considered characteristic of terrestrial upland birds, and the partially webbed feet characteristic of an aquatic animal. According to Weissman (1998), Audubon was able to reconcile the presence of both features by seeing frigatebirds as a transitional form between land and seabirds. While that relationship does not hold in light of modern analysis of anatomical and genetic characters, it is worth taking note that Audubon, like some of his contemporaries, was beginning to think in quasi-evolutionary terms well before Darwin.
Audubon, John James. 1835. Ornithological biography, or an account of the habits of the birds of the United States of America.” Vol. 3. Edinburgh: Adam & Charles Black.
Bush, Sarah E. et al. 2012. “Influence of Bill and Foot Morphology on the Ectoparasites of Barn Owls.” The Journal of Parasitology 98 (2): 256 – 261.
Clayton, Dale H. et al. “How Birds Combat Ectoparasites.” The Open Ornithology Journal 3: 41 – 71.
Weissmann, Gerald. 1998. Darwin’s Audubon: Science and the Liberal Imagination. Cambridge, MA: Perseus Publishing.