Drivers of decline and changed nest-site preference of the Baltic eider: an island-level analysis from south-western Finland

The recent large-scale decline of Eiders (Somateria mollissima) in the Baltic Sea is well documented by long-term population monitoring. One hypothesis to explain the decline is increased predation pressure during breeding, but because the decline has been steep and geographically broad, multiple drivers may be involved. Here we explore whether some of these hitherto unidentified drivers relate to the breeding habitat. To this end, we performed an analysis of long-term monitoring data from ca. 300 islands from the Archipelago Sea, SW Finland, representing five geographical sub-areas, during 1993-2012. We analysed population trends and whether nest-site preference changed over time. The results showed steep population declines in the sub-areas farthest from the mainland and a more moderate decline in the sub-area closest to the mainland. The presence of breeding gulls (Larus spp.) on the Eider breeding island had a consistently positive effect on Eider breeding numbers throughout the study period. In contrast, the presence of breeding White-tailed Sea Eagles (Haliaeetus albicilla) had a negative effect on breeding numbers, but only during the early study period when the Eagle breeding population was still small. Interestingly, the sign of the effect of the distance from the nearest neighbouring island on Eider breeding numbers changed over time. Eider breeding numbers positively correlated with island size and distance to neighbouring islands in the early period, whereas exposed large islands with islands nearby were positively associated with Eider breeding abundance during the late period. Future conservation efforts should shift from site-specific conservation to population-specific management due to the shift in nest-site preference. Furthermore, the positive influence of gulls warrants the adoption of conservation measures taking interspecific interactions into consideration.