ten Cate C. (1985) Functional aspects of head-scratching methods and other preening movements in birds. ARDEA 73 (1): 99-104

This article points to the relation between the use of one of two distinct head-scratching methods in birds (the overwing or indirect versus the underwing or direct method) and the postures adopted during other preening movements. Black-headed Gull, Mallard and Oystercatcher preen on the ground or on water and adopt postures preventing touching the substrate. Carrion Crow and Starling perch while preening and often keep wings and tail below belly level. The underwing head-scratching method, used by Black-headed Gull and Mallard, also seems best suited to prevent touching the ground, while in the overwing method, used by Carrion Crow, Starling and Oystercatcher, wings and tail are lowered. An experiment, in which Carrion Crows and Starlings preened on the ground, showed that the overwing head-scratching and also the stroking of the primaries in these birds (which is different from the primary stroking of the other species) are the postures in which touching the ground can be prevented less. These findings suggest that underwing head-scratching may, in some cases, have evolved to prevent touching the substrate. The Oystercatcher combines preening postures in which touching the ground is prevented with overwing head-scratching. This exception is discussed, as is the literature on function and evolution of the head-scratching methods.