BREWSTER : BIRDS OF THE CAPE REGION, LOWER CALIFORNIA. 85 



they furnish a nicely graduated series connecting those most heavily barred 

 with the nearly plain-tailed type of lucamnus. As the Arizona specimens of 

 culurm show quite as wide a range of variation with respect to the tail niark- 

 in^^s as do Mr. Frazar's birds, and as none of the other characters originally 

 ascribed to lucasanus prove more satisfactory or constant, it is evident that 

 the type of this supposed subspecies represents a mere accidental or extreme 

 variation of a form which, as already stated, does not normally differ in either 

 color or markings from Buteo borealis calurus of the western United States. In 

 other words, all the Red-tailed Hawks thus far found in the St. Lucas Region 

 are one and the same thing, and if they are to be separated from calurus — in 

 which case they must bear the name lucasanus — it must be by size alone. 

 The difference in this respect is so triiiiug that I cannot think it worth special 

 recognition. 



None of the Lower California or Arizona specimens resemble at all closely 

 B. b. socorroensis, but I have two adult birds (Xo. 26,206, Jan. 8, 1887 and No. 

 26,207 Feb. 16, 1S83) taken at Xicasio, California, by Mr. C. A. Allen, which 

 differ from this form, as represented by the three U. S. Nat. Museum examples 

 before me, only in having somewhat more black on the jugulum and throat 

 and in being slightly larger. Another specimen (No. 24,780) in my collec- 

 tion from Alamos, western Mexico, is generally similar, but has more black 

 spotting on the abdomen, and the breast and thighs are deeper colored. I see 

 no alternative but to refer both this and the two Nicasio specimens to socoi-- 

 roensis. Such reference need not be prejudicial to the subspecific standing of 

 the latter, for there is no reason why this bird, even if confined to Socorro 

 Lsland during the breeding season, should not wander, at other times of the 

 year, as far as Alamos, or even Nicasio. 



B. costaricensis differs so very widely in coloring from any of the phases of 

 calurus as not to require comparison in this connection. 



In the following tables of measurements no birds not fully adult (i. e. red- 

 tailed) are included : — 



Buteo borealis socorroensis Ridgw. 



117,5001 $ Socorro Isl., Mex. Mar. 8, '89 16.76 9.53 3.80 1.95 1.73 1.43 1.03 .90 



1 Collection U. S. Nat. Museum. Type, 50,761. 



2 Collection William Brewster. 



