Global population status of the migratory Holarctic species Arctic Skua <i>Stercorarius parasiticus</i>

Summary Data on species’ demography are essential to detect changes in population size, identify drivers of population change, motivate conservation plans, or evaluate the effectiveness of management. The Arctic Skua Stercorarius parasiticus is a seabird with a circumpolar distribution, which is listed as “Endangered” on the International Union for Conservation of Nature (IUCN) European Red List, although listed as “Least Concern” globally. It is both a predator and a kleptoparasite reliant on marine and terrestrial prey, and undertakes migrations from northern breeding habitats to temperate and tropical marine environments. Most studies of breeding populations originate from North Atlantic populations where Arctic Skuas are primarily kleptoparasitic. However, a large proportion of the global breeding population occupies remote coastal and inland tundra of Arctic regions where Arctic Skuas are more generalist in foraging modality and the range of prey taken. Here, we collated and summed national/regional population estimates to provide an updated global estimate of breeding population size and trends. We reviewed drivers of population change and knowledge gaps, and their implications for the conservation of this species. We estimated a minimum breeding population of 185,131–395,315 pairs combining Alaska, Canada, Greenland, and Europe; we extrapolated that at least 40,000 pairs could be found in Asian Russia, where no estimates were available. We noted differential trends, with substantial declines in typically kleptoparasitic populations of the North Atlantic where data quality was higher, whereas for populations in the Nearctic, trend data were scarce and geographically restricted. Various threats were identified as potential drivers of population change, including bottom-up processes, fisheries, heat stress, and interspecific competition/predation. Given the large uncertainty around abundance and population trends for much of the Arctic Skua’s range, the current global conservation status of Least Concern may be better designated as “Data Deficient”, and we encourage the implementation of a range of approaches to improve monitoring of population trends and demography globally.