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

Bemmelen, Rob S.A. van; Clarke, Rohan H.; Pyle, Peter; Camphuysen, Kees


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-constrained to complete primary molt before the onset of prebreeding long-distance migration. By contrast, molt duration did not differ between age classes of Great Skuas. Adult Great Skuas may have overcome the time constraint by completely overlapping molt and postbreeding migration. Molt-migration overlap is generally rare in birds but may be feasible for Great Skuas given their shorter migration distance and low migration speed.