South Polar and Great Skuas: the timing of primary moult as an aid to identification

Published on 11 June 2013 in Main articles

By Dick Newell, Steve Howell and Dani López-Velasco

Abstract The plumages and moult cycles of immature large skuas are poorly known, and some first-year Great Skuas Stercorarius skua can closely resemble some South Polar Skuas S. maccormicki. The most challenging birds are dark individuals in August-October with no active primary moult, and faded/moulting birds in May-August, which may be either first-year Great or older immature South Polar Skuas. Ageing is generally possible by careful observation of plumage coloration and pattern, in combination with leg colour, primary shape and primary wear. The breeding seasons of the two species differ by six months, but the moulting periods of adults of each species overlap broadly with the moulting periods of first-cycle birds of the other species. We show that, for birds of known age, primary moult score is a valuable tool for identifying immature Great and South Polar Skuas. Our data also suggest that South Polar Skuas take c. 150 days to complete their primary moult, similar to the time taken by Great Skuas.

South Polar Skua, Lanzarote, Canary Islands, 13th September 2011. This second-cycle (or older) bird, just completing its primary moult shows a number of features indicative of South Polar, including smooth scalloping on the flanks and a pale collar. It would be rare for a Great Skua to have this primary moult score on this date. Note that some brown feathers are visible in the underwing-coverts. Pic by Juan Sagardía

South Polar Skua, Lanzarote, Canary Islands, 13th September 2011. This second-cycle (or older) bird, just completing its primary moult shows a number of features indicative of South Polar, including smooth scalloping on the flanks and a pale collar. It would be rare for a Great Skua to have this primary moult score on this date. Note that some brown feathers are visible in the underwing-coverts. Pic by Juan Sagardía

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