Clinal variation in the juvenal plumage of American Kestrels

The American Kestrel (Falco sparverius) is a sexually dichromatic falcon th at exhibits considerable individual plumage variability. For example, the a nterior 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 Kes trels (F. S. paulus) exceeds that accounted for by age. The objective of th is study was to search for geographic patterns in the variability of juvena l plumage, particularly those characteristics considered indicative of age. Nestling kestrels (n = 610) were examined prior to fledging during the 199 7 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 f orehead and crown feathers become more pronounced, especially in males. Mal e Southeastern American Kestrels differed from other males (F. s. sparveriu s) 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 dine extending across the species range in the Western Hemisphere, where tropical subspecies are small and have reduced dorsal barring. Both body si ze 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 expla in why males become less barred with maturity while females do not.