By Jean FranÃ§ois Blanc, Audrey Sternalski and Vincent Bretagnolle
Abstract The Marsh Harrier Circus aeruginosus is typically illustrated in field guides as a sexually dimorphic species, with several age classes identifiable by differences in plumage pattern and colour. In some populations, however, such as the one we studied in west-central France, the species can show extreme plumage variability in adult males and, to a lesser extent, in adult females. Our study population is markedly polymorphic, with highly distinct patterns of coloration and almost continuous individual variation between those different morphs. Barely a single adult male looks like a typical ‘field-guide male’. Since this plumage variability is independent of age and sex, it is almost impossible to age birds solely from their plumage, which contradicts the established view. We highlight the difficulties of ageing and sexing Marsh Harriers by plumage only, and advocate the recognition of this species as polymorphic, at least in some parts of its range.
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