Clinal variation in the juvenal plumage of American kestrels

The American Kestrel(Falco sparverius) is a sexually dichromatic falcon that exhibits
considerable individual plumage variability. For example, the anterior extent of the
black dorsal barring in juvenile males has been used throughout North America as one of
several aging criteria, but recent data demonstrate that the variability among individual
Southeastern American Kestrels(E S. paulus)exceeds that accounted for by age. The objective of this study was to search for geographic patterns in the variability of juvenal plumage, particularly those characteristics considered indicative of age. Nestling kestrels (n = 610)
were examined prior to fledging during the 1997 breeding season at nest box programs
across a large portion of the North American breeding range. From south to north (1) the
crown patches of both males and females become more completely rufous, and (2) shaft
streaks on forehead and crown feathers become more pronounced, especially in males. Male
Southeastern American Kestrels differed from other males (E s. sparverius) in that the anterior
extent of dorsal barring averaged less but was more variable. The variability observed
in North America appears to be part of a cline extending across the species range in the
Western Hemisphere, where tropical subspecies are small and have reduced dorsal barring.
Both body size and, especially in males, dorsal barring increases with increasing north and
south latitude. We suggest that this geographic pattern is adaptive in terms of thermoregulation, and that differences in the sex roles may explain why males become less barred with maturity while females do not.