4 
Melanism is not linked with age, as is shown by a brood of half-feathered 
nestlings in our collections Nos. 10455-57, from near Red Deer, Alberta, 
July 4, 1917, which is going directly from the natal-down into an unmis- 
takable melanotic plumage. 
Juvenile calurus in the light phase are not always strikingly different 
from juvenile borealis. The most distinctive character of this plumage is a 
considerable mixture of red in the greyish-brown background of the tail 
(Plate I, figure 7). This red, however, seems largely evanescent, for 
although it is very common in birds just leaving the nest it is rarer in 
fully grown, self-supporting birds, and probably fades quickly, as does the 
cream and ochreish washes of the white parts of the very young. 
Many associations of calurus, however, include birds of strong borealis 
appearance. The last calurus character to persist is the crossbarring on 
flags and tail. These bars are occasionally faint or incomplete in calurus, 
but are still rarer in pure borealis strains. It is likely that birds occur in 
the heart of the range of their respective subspecies, that cannot be identi- 
fied by anything except the probabilities of geography. 
BUTEO BOREALIS KRIDERI, KRIDER’S HAWK 
Single phased (See footnote, previous). Like borealis, but with large 
and in many cases striking extension of white below, above, and on head 
and tail. It is largely black and white, with little or no cream or rufous 
in any plumage except when just leaving the nest. 
This subspecies is something of a puzzle. It was first described from 
a juvenile bird (Plate II, figure 1). As it seems even more confusingly 
variable than calurus and few specimens of known parentage or in changing 
plumage have been studied, the progression of plumage with age has been 
assumed rather than demonstrated. Most of the other forms of the 
species are essentially alike during juvenility, but juvenile krideri seem as 
variable as the adults and it is yet uncertain just what are the adult forms 
of many young types of krideri. The distinctive character of krideri in 
any age is the large amount of white. In extreme cases the head, face, 
neck, and all below may be pure immaculate white. There may be much 
white on the scapular tips and the wing coverts, and the tail may be white 
with or without dark bars either broken or entire (Plate I, figures 9-15; 
Plate II, figures 1, 2, 3, 6). There is every combination of these characters 
with those of borealis, calurus, or harlani (Plate II, figure 3; Plate III, 
figure 6). 
The white tail seems to be the most persistent krideri character and 
many specimens appear to be straight borealis or calurus with strongly 
marked krideri tails (Plate II, figure 3). On the other hand occasional 
specimens occur in which the body is strongly krideri, whereas the tails 
are straight harlani (Plate III, figure 6). It thus seems that the white 
krideri tail is dominant relative to borealis and calurus, but recessive to 
harlani. As in other races of B. borealis many juvenile krideri, identified 
by their parents, carry little or no distinctive subspecific characters of 
their own. 
