Sexual Dimorphism and Food Habits in Three North American Accipiters

THE three widespread North American bird hawks, the Sharp-shinned Hawk (Accipiter striatus velo.x),Cooper's Hawk (A. cooperil), and the Goshawk (A. gentills atricapillu's) differ greatly in size.In the course of studies on weight, wing area, and skeletal proportions of these three species (Storer, 1955), it became apparent that although the females of all three species average larger than the males, the sexual difference in size was greatest in the smallest species, the Sharp-shinned Hawk, and least in the largest species, the Goshawk.To determine more precisely the degree of sexual dimorphism in these three species, I measured the wing length (arc) of study skins in the collection of The University of Michigan Museum of Zoology.The series used were large enough to provide both a good estimate of variation within the species and an accurate mean.All birds measured were in adult plumage and were collected in the region between Grafton, in extreme eastern North Dakota, and Point Pelee, Ontario, Canada.The sample of the Sharp-shinned Hawk consisted entirely of birds taken in Michigan; those of the other species included birds from most of the broader area.Mis-sexed accipiters, especially Goshawks, appear to be not infrequent in collections, and I strongly suspect that a few mis-sexed specimens have led Friedmann (1950: 150-152) and possibly others to describe the variation both in measurements and plumage as overlapping more than actually will be found to be the case.Much of the mis-sexing probably results from the collectors' mistaking the paired ovaries (usual in birds of this genus) for testes.In our collections, four Goshawks sexed as males by the collectors measure 348, 357, 360, and 361 mm in wing length.All were taken in the winter months (December to February) when the gonads Vol.