The avoidance of living in the vicinity of a top predator: the coexistence of the black stork and the white-tailed eagle in NE Poland

Abstract The black stork Ciconia nigra is a rare species of bird that inhabits old forests near wetlands. The early 21st century has brought a regress of its population in north-eastern Poland. We verified the assumption that an important reason for the observed changes was the colonization of the study area by white-tailed eagle Haliaeetus albicilla, because we observed a correlation between decreasing numbers of the black stork and increasing numbers of the eagle. Based on the long-term monitoring, we analyzed changes in the numbers and distribution of the black storks’ nests and compared the colonization of the study area by the white-tailed eagle in the most extensive forest complex in Poland. We found 42 occupied territories of storks and no eagles in 1989 within the study area. In 2019, there were just 23 stork pairs, but the population of eagles increased to 13. We found a correlation between the rate of decline of the black storks population and the increase in the white-tailed eagles. In the first half of study period, storks’ nests have been distributed randomly, after which they became clustered. The pattern of nests distribution was influenced by the increase in the population of the eagle over time, with this ensuring the aggregation of the nests of storks in places more distant from the nests of eagles. Similarly, the nearest neighbor distance of storks’ nests was dependent on distances from the nests of eagles and the shares of territory accounted for by forests. We surmise that changes in the population size of storks were induced mainly by the growth of population of eagles. Storks avoided occupying nests less than 4.6 km from the eagles’ nests. Our study suggests that the determining role can be played by fear of the predator. Overall, it must be concluded that the protection of top predators can affect other co-occurring species in unexpected ways. Higlights In the Augustów Forest (north-eastern Poland), the number of the black stork Ciconia nigra over 30 years has strongly declined and the distribution of its occupied nests has changed from almost solitary to clustered. The colonization of the study area by the white-tailed eagle Haliaeetus albicilla was a plausible reason for the observed changes. Storks avoided occupying nests less than 4.6 km from the white-tailed eagles’ nests, which can be defined as a safety distance. The determining role can be played by the fear of the predator more than by direct predation.