Timing and duration of primary molt in Northern Hemisphere skuas and jaegers

Abstract

ABSTRACT We compared the primary molt of the 4 species of skuas and jaegers (Stercorariidae) that breed in the Northern Hemisphere: Long-tailed Jaeger (Stercorarius longicaudus), Parasitic Jaeger (S. parasiticus), Pomarine Jaeger (S. pomarinus), and Great Skua (S. skua). We analyzed primary molt data of 1,573 individuals of multiple age classes, mostly collected from photographs taken at sea but also from museum specimens and beached individuals. Whereas molt duration generally increased with species' size, molt duration in Parasitic and Pomarine jaegers was surprisingly similar given their size difference. Larger species started primary molt earlier and showed more overlap with postbreeding migration, such that there was complete overlap in Great Skua but no overlap in Long-tailed Jaeger. Within jaeger species, the first primary molt cycle took longer than later molt cycles. We suggest that, unlike birds in their first primary molt cycle, birds in their second or subsequent primary molt cycles are time-c...