Anthropogenic influence on the interaction between the white-tailed eagle Haliaeetus albicilla and the common guillemot Uria aalge on the island of Stora Karlsö in the Baltic Sea

In 2020, the ‘anthropause’ due to the spread of the global pandemic COVID-19 interfered with the biotic interactions on the island of Stora Karlsö, in the Baltic Sea. The absence of tourists caused an increase of white-tailed eagle Haliaeetus albicilla which consequently disturbed the breeding common guillemot Uria aalge, that form the most important colony in the Baltic Sea. The effect of humans on these 2 species has been quantified again in 2021, to investigate whether the common guillemot - white-tailed eagle interaction had changed transitorily or permanently. The objective of this study has been reached with the collection of data regarding tourist presence, with the monitoring of the white-tailed eagle presence and disturbance on common guillemot, and with the calculation of the breeding performance of common guillemot. The partial return of tourists in 2021 brought some changes with respect to 2020: a decrease in the white-tailed eagle number, a decrease of white-tailed eagle disturbance on common guillemot, and an increase of the common guillemot breeding performance. In 2021, tourists influenced white-tailed eagle location around the island, and they caused a differentiated white-tailed eagle disturbance rate on common guillemot in different sub-colonies of the island. Based on these findings, touristic presence on Stora Karlsö has been confirmed to play a beneficial role for the common guillemot community. It also emerged the necessity to keep on monitoring the relationship between eagles, guillemots, and humans in order to pursue proper management and conservation strategies of the species within their ecosystem.