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

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.