The Cooper Hawks 



female, length: 457.2-508 (18.00-20.00); wing 247.6-279.4 (9.75-1 1. 00) ; tail 215.9-254 

 (8.50-10.00); bill from cere 17.8-20.3 (.70-. 80). 



Recognition Marks. — Crow size; adult heavily barred below with cinnamon- 

 rufous; young heavily striped on breast and sides with dark brown or dusky; top of 

 head blackish; tail long, rounded. Almost always distinguishable from A. velox by 

 greater size. 



Nesting. — Nest: Of sticks, with scant lining of green leaves, high in trees. 

 Sometimes a deserted Crow's nest is used. Eggs: 3 to 6; pale bluish white, sometimes 

 spotted with light brown; subspherical. Av. size 48.8 x 38.6 (1.92 x 1.52). Season: 

 April 20-May; one brood. 



Range of Accipiter cooperi. — North America. Breeds from Mexico and southern 

 border of the United States north to the southern Canadian provinces and southern 

 Keewatin. Winters on the Pacific Coast from British Columbia and from the central 

 eastern states south to Costa Rica. 



Range of A. c. cooperi. — The eastern and northern portion of the range above 

 defined — line of demarcation from A. c. mexicanus undetermined. 



Distribution in California. — Common resident at all levels up into Transition. 

 Presumably confined to region north of Tehachipe. More in evidence in winter, and 

 numbers possibly augmented from the North. 



Authorities. — Gambel (Astur cooperii), Proc. Acad. Nat. Sci. Phila., vol. iii.. 

 1846, p. 46 (Calif.); Fisher, Hawks and Owls of the U. S., 1893, p. 38, pi. 5 (food); 

 Grinnell, Univ. Calif. Publ. Zool., vol. v., 1908, p. 57 (San Bernardino Mts.; desc. 

 nest and eggs; habits); ibid., vol. xii., 1914, p. 124 (Colorado Valley; desc. nest and 

 eggs, etc.); Willett, Pac. Coast Avifauna, no. 7, 1912, p. 46 (s. Calif.; occurrence, nesting 

 dates, etc.); Howell, Pac. Coast Avifauna, no. 12, 1917, p. 54 (occurrence on Santa 

 Cruz Id.). 



No. 328a Mexican Cooper Hawk 



A. O. U. No. 333, part. Accipiter cooperi mexicanus Swainson. 



Description. — Similar to A. c. cooperi, but averaging smaller, the wing averaging 

 about 6 mm. shorter than that of cooperi. Also "female with markings of lower parts 

 denser and rather deeper in color; more rufous on thighs" (Kirke Swann). 



Nesting. — As in A. c. cooperi. Eggs: Averaging much smaller. Av. of 21 

 specimens from southern California: 46.2 x 36.6 (1.82 x 1.44). 



Remarks. — Dr. Grinnell in the Colorado River Report [U. of C. Pub. Zool., 

 vol. 12, no. 4 (1914), pp. 124, 125] was perhaps the first to call our attention to the 

 small size of Cooper Hawks' eggs taken in the Southwest. Having the curiosity to 

 check up on this point, I find that 143 eggs from: Ohio (Fordyce), New York (Rath- 

 bun), Pennsylvania (Burns), and Massachusetts (Bent), average mm. 49.02 x 38.82; 

 while 21 eggs from southern California average mm. 46.2 x 36.6. This difference may 

 not appeal to the layman as being very significant; but by application of the well- 



4Kab^ ... 



known formula, (for determining the cubical content of a prolate spheroid, 



which, for practical purposes, an egg is assumed to be), we discover an average difference 

 in bulk, or weight, of nineteen per cent of the smaller. Whether, indeed, this indubi- 

 table reduction in the size of eggs from the Southwest may be correlated with the alleged 

 smaller dimensions of Accipiter cooperi mexicanus, remains to be seen; but the point 



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