207 
7309. falco mexicanus Schleg., Abh. Geb. Zool. W. North 
heft ii1., p. 15 (1844). [ex Licht.—Mexico America, from 
= Monterey. | S. Bae: 
Prairie Falcon. Columbia to 
S. Mexico. 
Wing 3 305-328, 2 330-350 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. 
F. Larger; length about 18-24 in. (Sub. gen. 
Hierofalco Cuvier). 
7*310. Falco rusticolus rusticolus! Linn., S.N.,i., N. Sweden and 
p- 88 (1758). [Sweden.] Norway ; 
Norwegian Gyrtalcon. Lapland ; 
N. Russia ; 
[Grey-backed form]. Wing ¢g 345-370, S. in winter to 
2 388-405 mm. ; head blackish grey ; above 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. 7. rusticolus, F. ry. islandus, and F. r. alascanus. 
Possibly to these may be added F. rv. uralensis, if we could ascertain that the grey- 
backed Asiatic birds had a fixed breeding range south of their albinistic northern 
relatives, but this we do not know. Bering Island forms a remarkable instance, 
according to Stejneger, moreover, of a colony of smallish white 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. 
