A rare dark morph in the Canadian Arctic raises questions about molting and polymorphism in Long-tailed Jaeger (Stercorarius longicaudus)

Abstract

ABSTRACT The existence of a dark morph in adult Long-tailed Jaeger (Stercorarius longicaudus) is still unclear and debated. For this species, dark plumages occur in juveniles and immatures, but no dark morph adults have yet been unequivocally confirmed. Ambiguous reports of presumed dark morph adults and lack of information on molt progression of immatures generate confusion regarding this issue. Here we describe observations conducted in the Canadian Arctic (Bylot Island) and show that Long-tailed Jaegers can apparently exhibit dark morph adult-like plumage (i.e., individuals showing some obvious criteria associated with definitive alternate ‘adult breeding' plumage). Careful examination of those sightings, however, also revealed immature characteristics, indicating the high variability of the first prealternate molt in this species. Well-documented observations of rare plumages like the one reported in this study are needed to elucidate molting strategies and polymorphism in Long-tailed Jaeger. Received...