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.

To assess mitigation measures for habitat loss, it is important to understand the function of artificial wetlands as alternative wader habitat. In the Netherlands since 2016, a 1,000-ha freshwater archipelago called Marker Wadden has been constructed 3 km offshore in lake Markermeer. This artificial ecosystem provided a unique opportunity to study the impacts of construction designs and development of habitats on waders. Over a seven-year period, we studied Pied Avocets Recurvirostra avosetta because they used habitats in the constructed basins right from the start for feeding, and they are of conservation concern in the Netherlands. Between 2017 and 2023 we counted staging Pied Avocets and monitored habitat development in basins that varied in the time of construction (age) and design (closed or open, i.e. with breached levees connecting to the surrounding lake). Annual habitat changes were assessed with satellite images, and with pictures taken monthly at 20 fixed locations. With negative binomial zero-inflated models, we tested the effects of habitat characteristics, basin age, size and design, and raptor presence on bird numbers. The maximum number of Pied Avocets declined from >500 in 2017 to <100 in 2023. Basin age captured all habitat variation caused by natural succession and had the strongest negative effect on bird numbers. Open basins, allowing permanent flooding, were used less than closed basins. Avocet numbers remained highest in basins containing a mosaic of water and mud, with no or sparse vegetation. We concluded that Pied Avocets decreased due to two reinforcing processes: vegetation succession and breaching of levees leading to a rise in water level. Future developments targeting Pied Avocet conservation in freshwater wetlands should create and maintain shallow and muddy habitats with the closed basin and size concept as guidelines.