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.
Order your back issue, or subscribe today