1914] Grinnell: Mammals and Birds of the Colorado Valley 125 



in millimeters 45.0 by 37.2 and 45.6 by 36.5. The average for the 

 species, as given by Bendire (1892, p. 195). is 49.0 by 38.5. This is 

 another instance of diminution in the size of the eggs in the southern 

 part of the range of a species. It is not paralleled in this ease by 

 decrease in size of the birds themselves. The three specimens of 

 Cooper hawk obtained from the Colorado Valley (nos. 12692-12f>94) 

 are in no respect smaller than individuals taken in the breeding season 

 in the Transition zone, where large eggs are laid. The explanation 

 possibly lies in the demands of the developing embryo for a larger 

 supply of heat-producing food in the colder zone. Even though this 

 species incubates closely, the temperature of the egg after deposition 

 is doubtless affected to some extent by that of the outside air. 



There is in the Museum an immature female specimen of this hawk 

 (no. 4378) taken by J. G. Cooper at Fort Mohave. April 27, 1861. 



Buteo borealis calurus Cassin 

 Western Red-tailed Hawk 

 This hawk proved to be regularly distributed along the valley of 

 the Colorado, both in its timbered portions and where it cut through 

 mountains. Yet the species was not nearly as numerously represented 

 anywhere along the river as it is ordinarily on the Pacific slope of 

 California. Exact places of observation were : five miles below Nee. lies ■ 

 both sides of the river at The Needles; above Bill Williams River; 

 Ehrenberg ; ten miles below Cibola ; twenty miles north of Picacho : 

 four miles south of Potholes. All adults seen by us closely enough for 

 determination in this regard, were in the light phase of plumage 

 coloration. There is. however, in the Museum an adult female speci- 

 men (no. 4372) taken by J. G. Cooper at Fort Mohave, April 3. 1861. 

 which is distinctly in the dark phase. There is also a specimen (no. 

 4373), same place and collector, taken March 7, 1861, which is in 

 immature plumage, and at least as heavily marked as skins in like 

 stage from the California coast region. 



Falco mexicanus Schlegel 



Prairie Falcon 



Two pairs were seen alone: the steep rock wall of the canon imme- 

 diately below T the mouth of Bill Williams River. An evident aerie 

 was noted some 200 feet above the water in a hole near the top of the 

 face of a cliff. 



