Temporal changes in size of Sharp-shinned Hawks during fall migration at Cedar Grove, Wisconsin

Abstract

We measured wing chord, tail length and mass and estimated levels of subcutaneous fat deposits of Sharp-shinned Hawks (Accipiter striatus) trapped at Cedar Grove, Wisconsin. Wing chord, tail length, and mass all showed a sudden increase beginning about 10 October in adults and juveniles and in males and females despite the fact that adults migrate about 20 d later than juveniles and males migrate later than females. Migrants captured at Cedar Grove late in the season did not differ significantly in wing chord or tail length from migrants captured at the Goshutes in eastern Nevada. We suggest that an increasing proportion of the birds captured at Cedar Grove after 10 October are of western origin because winds are more westerly in October than in September. The greater mass of Cedar Grove birds may be because of greater availability of prey than in the Goshutes or because the lower mass of birds in the Goshutes is a facultative adaptation for easier passage through the arid west.