207 



|309. Falco mexicanus Schleg., Abh. Geb. Zool. 

 heftiii., p. 15 (1844). [ex Uchi.— Mexico 

 = Monterey.] 

 Prairie Falcon. 



Wing S 305-328, ? 330-350 mm. ; above 

 brown, with pale fulvous margins ; crown 

 and nape with whitish margins ; tail tipped 

 with white and all but centre pair of 

 feathers barred on inner webs with whitish, 

 but outer webs scarcely spotted ; below 

 white, chest streaked and breast and belly- 

 spotted with brown, more in form of bars 

 on flanks. 



W. North 

 America, from 

 S. Brit. 

 Columbia to 

 S. Mexico. 



F. Larger ; length about 18-24 in. (Sub. gen. 

 Hierofalco Cuvier). 



t*310. Falco rusticolus rusticoliis'^ Linn., S.N., i., N. Sweden and 



p. 88 (1758). iSweden.-] 

 Norwegian Gyrfalcon. 



[Grey-backed 

 ? 388-405 mm. 



form]. Wing ^ 345-370, 

 head blackisli grey ; above 



Norway ; 

 Lapland ; 

 N. Russia ; 

 S. in winter to 

 Middle 



dark slate grey barred with pale bluish Europe. 



1 A close study of the Gyrfalcons in Europe and America has led me to the 

 conclusion that there is only one species, which is practically circumpolar. In its 

 primitive plumage it was evidently slate-backed, like the typical form {rusticolus), 

 but its habitat has induced an albinistic plumage which has resulted in the larger 

 and more powerful form we call candicans. This albinistic plumage, although 

 general, is not constant, hence we get in Greenland, and elsewhere, a percentage of 

 primitive grey-backed birds [holboelli of Sharpe, islandus or gyrfalco of other authors), 

 as well as a juvenile plumage which is identical with that of the primitive phase. 

 This albinistic phase has naturally its reflex in a melanistic phase, which is most 

 common in Labrador [obsoletus of Gmelin, labradora of Audubon) but also occurs in 

 Greenland. Where, however, the range of this species extends southward of the 

 limits of perpetual snow in such a fixed degree that a permanent colony is established, 

 we find the primitive phase coming out as the only plumage ; then we get a race 

 which may justly be named as distinguishable from the common stock of the boreal 

 species. Such examples are F. r. rusticolus. F. r. islandus, and F. r. alascanus. 

 Possibly to these may be added F. r. uralensis, if we could ascertain that the grey- 

 backed Asiatic birds had a fixed breeding range south of their albinistic northern 

 relatives, but tliis we do not know. Bering Island forms a remarkable instance, 

 according to Stejneger, moreover, of a colony of smallish wliite birds breeding far 

 to the southward of their usual range. Yet, on Bering Island occur larger white 

 birds and also grey-backed Siberian birds (Tring Mus.), but only in winter as far as 

 we know. 



