WESTERN RED-TAILED HAWK 1G7 



stub, ill which a goldeneye had a set of 10 eggs and a house wren a 

 set of 7 eggs (pi. 48). 



Eggs. — The eggs of Krider's hawk are practically indistinguish- 

 able from those of other redtails, though Mr. Peabody (1895) says 

 that some of them are more like red-shouldered hawk's eggs in their 

 markings. The measurements of 52 eggs average 59.8 by 49.9 milli- 

 meters; the eggs showing the four extremes measure 63.4 by 48.3, 

 G2.7 by 50.1, 55.7 by 44.6, and 58.4 by 44.2 millimeters. 



The sequence of molts and plumages is the same as in other red- 

 tails, but the racial characters are always evident. Its food habits, 

 general behavior, and voice are all similar. It migrates away from 

 the northern portion of its range for the winter. A. G. Lawrence 

 tells me that it is very rare in southern Manitoba, departing in Sep- 

 tember and returning in April. 



BUTEO BOREALIS CALURUS Cassin 

 WESTERN RED-TAILED HAWK 



HABITS 



From the western edge of the Great Plains westward this very 

 variable form of the red-tailed hawk breeds from southeastern 

 Alaska and central western Mackenzie southward to Mexico. Its 

 chief characters are its interesting color phases, varying from a light 

 phase, which is practically indistinguishable from the eastern bird, 

 to a melanistic phase, which is wholly dark sooty brown, except for 

 the red tail. Between these two extremes are numerous intermedi- 

 ate plumages, to be referred to later. The immature bird is con- 

 sistently darker, more heavily streaked below and with a more or 

 less distinctly streaked throat, and more spotted tibiae. 



Because of its similarity in the light phase to the eastern bird, 

 some very good ornithologists think that the western bird should 

 not be given a separate name. The color phases are no less variable 

 than, or strikingly different from, those that occur in Swainson's 

 hawk, the ferruginous roughleg, and some other hawks. 



Spring. — Throughout the northern portion of its range the west- 

 ern redtail is migratory. M. P. Skinner's records for 10 years in 

 Yellowstone National Park show that it was first seen there very 

 regularly between March 29 and April 3, departing again about 

 the middle of October. He thinks its arrival in spring is deiDendent 

 on the appearance of the ground squirrels, wdiich come out of hiber- 

 nation about the first of April. 



That these hawks know where to find a good food supply while 

 migrating is shown by their spring visits to the Farallon Islands, 

 of which Walter E. Bryant (1888) writes: "Every spring the island 

 is visited by numbers of these hawks. In 1882 they came in April, 

 about the time of the arrival of the murres, leaving again in May. 



