Skuas (Stercorarius spp.) moult body feathers during both the breeding and inter-breeding periods: implications for stable isotope investigations in seabirds
Résumé
Seabirds are mostly thought to moult during the inter-breeding period and the isotopic
values of their feathers are often therefore assumed to relate to their assimilated diet during
such periods. We observed Brown Skuas Stercorarius antarcticus lonnbergi and South
Polar Skuas Stercorarius maccormicki moulting on a breeding site at King George Island,
Antarctica. This raises concerns about the reliability of using stable isotopes in feathers
to infer feeding localities of birds during the inter-breeding period. We analysed the
d13C and d15N values of growing and fully grown body feathers collected from the same
individuals. For both species, d13C values of growing feathers indicated feeding areas in
the Antarctic zone (breeding grounds), whereas most fully grown feathers (100% for
South Polar Skuas and 93.3% for Brown Skuas) could be assigned to northern latitudes
(non-breeding grounds). However, a few fully grown body feathers of Brown Skuas
(6.7% of the feathers, belonging to two birds) showed isotopic values that indicated
moult in the Antarctic zone. As the growth period of those feathers was unknown, they
could not be used with confidence to depict the foraging behaviour of the birds during
the non-breeding period. Although precautions must be taken when inferring dietary
information from feathers in seabirds where the moulting pattern is unknown, this study
shows that if the development stage of a feather (growing/fully grown) is identified, then
dietary information from both breeding and non-breeding seasons can be obtained on
the same individual birds.