2 
BUTEO BOREALIS, RED-TAILED HAWK 
The Red-tailed Hawk inhabits North America from the tree limit in 
the north to northern Mexico. It migrates from the northern regions in 
winter, but seems to breed throughout its range. In the east it is mono- 
chromatic; in the west it is dichromatic, one phase of which is completely 
melanotic; and through the middle of the continent occur two types of 
variants, known as Krider’s and Harlan’s Hawks, which seem largely of 
sporadic occurrence and without well-marked distribution. 
There is no appreciable difference in plumage in the sexes of the 
species , 1 but in most individuals there is one decided change with age. 
Juveniles are usually distinct in coloration from adults, but correlation of 
the juvenile with its proper adult plumage has been largely a matter of 
intuition, guessw T ork, or judgment rather than demonstration, and false 
ideas on the subject are current. A study of the material bearing on the 
subject shows that there is only one abrupt change of plumage pattern in 
the life history of the individual after leaving the nest. The juvenile 
plumage is assumed in the nest and is retained until the next midsummer 
moult, when the adult plumage is assumed at approximately fifteen months 
of age. After this there is no appreciable progression of plumages or colour 
changes year after year, except that due to fading of pigment or wear of 
feather structure. In a large number of moulting birds we find them in 
every case either going directly from juvenile into a highly developed 
adult plumage or changing from an adult plumage to another practically 
identical with it. There is no indication of the generally assumed con- 
tinued progression from a less to a more mature type after the adult plumage 
has been once assumed. This greatly simplifies matters as it leaves two 
usually distinct sets of plumage to correlate within each race, instead of a 
multitude of slight progressions. On the other hand, the fact that many 
age plumages have been assumed to occur indicates the extreme variability 
of the individuals for which segregation into racial groups has been attempted. 
Although in most normal plumages birds of the year are easily dis- 
tinguished from adults, there are certain phases (melanotic phases, for 
instance) in which determinations of age by plumage becomes difficult, 
if not impossible. In general, the tail feathers of juveniles are narrower 
and more pointed than freshly grown feathers of the adult. However, 
numerous undoubted adults have the ends of their tail feathers reduced 
by wear or otherwise to quite juvenile outlines and in some cases it takes 
nice judgment to decide from this criterion whether particular birds are 
juvenile or adult. This is particularly true of some forms of krideri and 
introduces an element of doubt when transition material is not available 
as a check. 
There does not seem to be any evidence that the Red-tailed Hawk 
ever breeds in the juvenile plumage. It probably does not nest until the 
spring after the adult plumage is obtained, when the bird is about two 
years old. Nesting birds, therefore, can normally be assumed to be finally 
adult in plumage. 
1 Dr. L. B. Biahop informa the writer that “Male krideri is always paler than the female, and all very fine 
birds (typical) I have seen were males.” This may well be so, but the form is so variable that the writer has not 
been able to verify the statement beyond question by his own experience. 
