Primary moult of great skuas on the breeding grounds, with special attention to the Faroe Islands

Primary moult in birds demands energy and lowers flight ability. Therefore, timing of primary moult does usually not overlap with breeding or migration. Great skuas are exceptional, as they may start primary feather moult by the end of the breeding season and continue it during autumn migration. However, sixteen publications from 1921-2018 show variation in the timing and location of the onset of primary moult and it is unclear if this reflects geographical or temporal variation. Moreover, a quantification of the extent of primary moult on the breeding grounds is lacking. Here, we show for great skuas on the Faroe Islands, that they can moult the innermost four primaries (p1-4) on their breeding grounds. We collected 306 shed primaries in three areas: Skúvoy (2012 and 2013), Saksun and Svínoy (2013). We identified the primary types (p1-4) of these feathers based on morphometry and comparison with reference specimens, and found that shed feathers comprised 51% p1, 36% p2, 12% p3 and 1% p4. These frequencies did not differ significantly between the three areas, nor on Skúvoy between 2012 and 2013. Remarkably, they also did not differ between the breeding colony and the club site on Skúvoy, whereas this was expected as club sites are also visited by immatures, which start moult earlier than breeding adults. On Skúvoy, we trapped 19 adults on the nest, marked their inner 5 primaries on both wings (190 individual feathers), and retrieved 6 (3.2%) of these primaries in the field after shedding. Based on these marked feathers we estimate that at least 45% of the 300 breeding great skuas on Skúvoy moult at least p1 there. An inquiry among colleagues showed that primary moult occurs throughout the breeding range, and seemingly to a larger extent in the south (Iceland, Faroes, Shetland) than in the north (Norway, Jan Mayen, Bjørnøya, Svalbard) by mid-August. We discuss several possible explanations for these patterns: variation in the timing of the breeding season, the proportion of failed and non-breeders, and the food situation. Such variation could explain on the one hand the discrepancies between literature sources and on the other hand the wide range in reported start dates of primary moult. figure 1. Great skuas at the club site at Vatndalsvatn, Skúvoy, Faroe Islands, 7 June 2013. Photo: Kees Schreven Grote jagers op de soos bij Vatndalsvatn, Skúvoy, Faeröer, op 7 juni 2013.