FAMILY ACCIPITRIDAE 209 



or buffy white, with 9 or 10 bars of dull black ; throat white, streaked 

 with grayish brown to dull black ; sides of the head dull black ; upper 

 breast grayish brown to light russet, with narrow black shaft lines ; 

 rest of under surface buffy white, varying from nearly immaculate, 

 to spotted and barred irregularly with grayish brown to tawny ; tibia 

 barred with cinnamon ; under wing coverts white to buffy white with 

 the longer outer ones dark gray to dull black, forming a distinct spot ; 

 elsewhere spotted lightly with grayish brown to cinnamon. 



Rufous phase, entire under surface washed with light brown, 

 barred narrowly with russet ; concealed nape patch buffy. 



Black phase, body throughout dull black to brownish black; 

 under tail coverts buffy, barred with fuscous. 



Immature, blackish above, with forehead extensively white ; crown 

 and hindneck broadly streaked with white, and rest of upper surface 

 heavily and irregularly spotted with white; below buffy white, with 

 a fuscous area on breast. Birds in the first year vary from this to 

 individuals with under surface fuscous-black spotted with tawny. 



As the description indicates there is much variation in color. In 

 life the lighter individuals are marked by the dark breast. The 

 blacker ones suggest the zone-tailed hawk, but usually they will 

 be seen in migrant flocks with their lighter colored companions, while 

 the other species is found alone or in pairs, and is very rare. In the 

 hand it will be seen that 3 outer primaries are notched near the tip, 

 in which Swainson's hawk resembles the white-tailed hawk. From 

 that species Swainson's hawk differs in having the tarsus definitely 

 less than 80 mm. long, while in the other this measures 85 to 95 mm. 



Measurements (from Friedmann, U. S. Nat. Mus. Bull. 50, pt. 10, 

 1950, pp. 297-298).— Males, wing 362-406(383.6), tail 185-214 

 (204.6), culmen from cere 20.5-24.9 (22.1), tarsus 63.1-72.6 (68.2) 

 mm. 



Females, wing 375-427 (404.6), tail 193.6-234.0 (214.6), culmen 

 from cere 20.5-25.7 (23.7), tarsus 61.5-76.4 (70.6) mm. 



Migrant from the north. Seen usually in flocks, en route to or 

 from a winter range in South America. 



The southward flight begins in the latter part of September (Sept. 

 28, 1940) and continues through October (Oct. 22, 1911). The north- 

 ward movement starts in early February (Feb. 3, 1952), with the 

 main migration in March and the beginning of April (large flocks 

 Apr. 2, 1950). A few continue to pass through early April. Some 

 remain on the isthmus during the northern winter, as indicated by 

 2 skins in the U. S. National Museum, one taken near Gatun, Canal 



