Sexual Dimorphism and Food Habits in Three North American Accipiters

Abstract

THE three widespread North American bird hawks, the Sharp-shinned Hawk (Accipiter striatus velox), Cooper's Hawk (A. cooperii), and the Goshawk (A. gentilis atricapillus) 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 are small, and all have the heavy black streaking on the under parts, which is usually found in adult females. A fifth Goshawk, sexed as a female, measures 324 mm in wing length, has the fine barring characteristic of adult males, and lacks heavy black streaking. These five birds were omitted from the samples, and even so the coefficient of variation is over 10 per cent larger for both sex groups of the Goshawk thlan for those of the other species (Table 1). A Cooper's Hawk sexed as a male but measuring 261 mm in wing length and having the brownish plumage of the upper parts characteristic of adult females was also omitted from the data. The measurement data are summarized in Tables 1 and 2. Judging froim the difference between the means for males and for females and from