Head Scratching

Left: A Swainsons Warbler scratches under the wing.
Right: A Northern Parula scratches over the wing.
ink draw by Shahid Naeem
Head scratching is so essential to birds that even one-legged individuals will attempt it. As far as we can tell, it has several functions related to plumage maintenance. Since a preening bird cannot reach its head with its beak, scratching helps to spread preen oil there. Indeed, some species gather preen oil on the bill, scrape the bill with the foot, and then scratch the head. Head scratching may also remove molted feathers. The area of the head most frequently scratched is near the ear, and it has been suggested that the behavior is associated with pressure changes in the eustachian tubes. This, however, seems counterintuitive since claws are not inserted inside. But chronic ear scratching suggests that there may be another function in addition to spreading preen oil and cleaning. It could be
removing ectoparasites (those that live on the outside of the host) and their eggs, something that is done with the bill on other parts of the body.

The motions used for head scratching are quite ritualized, and vary from species to species. For instance, among North American wood warblers, seven species, including the Tennessee Warbler, Mourning Warbler, and Ovenbird, scratch their heads by directly raising a leg toward the front. In contrast, 31 wood warbler species, including all of the genus Dendroica (e.g., Golden-winged, Chestnut-sided, Yellow, Pine, Black-throated Green, and other warblers), are "overwing scratchers." They scratch their heads by extending the leg over a drooping wing that is held close to the body.

Within a species, the pattern of scratching is constant, but it is not related to the taxonomy of the warblers. Of two closely related species, one may scratch under, and the other over, the wing. There is, however, an intriguing and as yet unexplained correlation between the ecology of the warblers and their scratching method. Species that dwell mostly on the ground tend to scratch under the wing; those that are primarily arboreal, over the wing. Perhaps underwing scratching helps to keep the wings of ground-dwelling birds clean.
SEE: Bathing and Dusting; Anting; Disease and Parasitism.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.