Abstract

Regular monitoring of breeding population abundance and environmental factors related to the nesting habitat has proven fundamental for seabird conservation. Harmony Point (Nelson Island) is an Antarctic Specially Protected Area (ASPA) designated to manage and protect the high biological value of seabirds' richness and abundance. However, due to the remote location of their breeding sites, many species lack updated population counts. Skuas (Catharacta sp.) exhibited a two-decade gap since the last census was conducted in Harmony Point. The abundance and spatial distribution of the nests of skuas were studied during the austral summer of 2019/2020. Through an exhaustive search, we counted and mapped active nests. Nesting habitat was assessed by the use of an unmanned aerial vehicle to take aerial pictures and build an orthomosaic image to determine vegetation in the area. Additionally, a digital elevation model was built to calculate a series of geomorphological-related variables. Suitability models were used to estimate the importance of variables to the nesting of skuas. A total of 71 brown skua (Catharacta antarctica) and 3 south polar skua (C. maccormicki) nests were counted. Two of the seven variables (terrain slope and vegetation cover) accounted for 57.5% +/- 14.1% of the models' variability; sun radiation incidence, and wind shielding were of secondary importance. Water flow accumulation, distance from penguin colonies, and terrain elevation were the least important variables. Skuas selected for nesting flat terrains (slope < 10 degrees) with a vegetation cover of above 20%, slightly higher sun incidence (270 to 280 kW/h), and intermediary windshielding (45% to 55% of exposition). Considering previous estimates, the skua species at Harmony Point has kept an apparently stable population size over the last 25 years. However, expected changes in nesting habitat availability, i.e., increased snow-free area, increased wind intensity, changes in vegetation cover, and reduction of penguin populations, might change population size in the mid to long term.