Living in the danger zone: Impact of increased predation on the endangered common eider

Predation is a fundamental ecological force shaping the life histories of prey species. It influences reproductive strategies, habitat selection, and population dynamics across taxa. In the Baltic Sea archipelago, humans have profoundly altered predator–prey interactions by facilitating the recovery of a native apex predator, the white-tailed eagle (Haliaeetus albicilla), and by spreading the invasive mammalian predators the American mink (Neogale vison) and raccoon dog (Nyctereutes procyonoides). In long-lived species such as sea ducks, adult survival is typically the strongest demographic determinant of population growth, making predation on breeding females particularly consequential. Nest predation, however, remains the leading cause of reproductive failure in birds and can significantly reduce fecundity. Importantly, predation pressure on adults and nests may influence breeding behaviour in different ways, and according to life-history theory, long-lived species should prioritize their own survival over current reproductive investment, as they have multiple opportunities to reproduce. These dual pressures—on both adults and nests—may shape reproductive decisions in complex ways, yet we still lack a comprehensive understanding of how prey respond to different types of predation risk across spatial scales and through time. Key knowledge gaps include assessing if adult and nest predation differentially influence demographics, how females weight personal versus public information under variable predation risk, and the management effort needed to make invasive predator removal durable. Understanding prey responses to distinct predation pressures is essential for predicting population trajectories and species resilience, and guiding conservation strategies in rapidly changing ecosystems. In this thesis, I examine how a shining Baltic predator regime, characterized by white-tailed eagle recovery and the spread of invasive mink and raccoon dog, affects the endangered common eider (Somateria mollissima), focusing on demographic and behavioural responses of breeding females to elevated predation. As a ground-nesting species with high site fidelity relying mostly on crypsis, breeding female eiders are particularly vulnerable to predation, making them an ideal model for studying predator-induced reproductive constraints. Drawing on long-term data from two subpopulations (Tvärminne 1991-2023; Velkua 1991-2020), I examine how predation pressure affects spatiotemporal variation in clutch size and nest success across the subpopulations, how predation shapes breeding dispersal decisions in Tvärminne, and evaluate the effectiveness of invasive predator removal in Tvärminne. I use long-term individual-based monitoring of breeding females, complemented by island-level surveys. The longitudinal data integrate female characteristics (e.g., age, body condition, clutch size, breeding dispersal), nest cover and nest density, multi-scale predation risk indices (nest and adult predation risk, white-tailed eagle abundance), numbers of invasive predators removed, colony habitat features (e.g., forest cover, exposure), as well as climatic and phenological variables. The three chapters together assess how predation by native and invasive predators shapes reproductive strategies, female survival, and the potential for conservation interventions in a rapidly changing archipelago ecosystem. The results showed that predation by both white-tailed eagles and invasive mink and raccoon dogs was the dominant correlate of reproductive performance. Nest predation risk varied across space and time and was strongly associated with eagle predation pressure and habitat features (island forest cover and exposure), with contrasting associations with clutch size and breeding success between subpopulations. Females increased their breeding dispersal distances in response to predation cues, following a win–stay, lose–switch decision rule informed by both personal experience and public information, as indexed by island-specific nest predation risk. Switching breeding islands was more common in years with higher eagle abundance, and less common among older females. The removal of invasive predators significantly reduced the mortality of adult females, increased nest success, and enhanced breeding propensity, despite high predation pressure from the native white-tailed eagle. These findings highlight the importance of predator identity and hunting mode in shaping reproductive outcomes under habitat-mediated risk in long-lived prey with limited reproductive flexibility. Accordingly, conservation strategies need to target predator-specific pressures in their habitat context. Notably, contrasting fecundity trends between subpopulations—driven by differences in predator pressure—highlight the importance of site-specific management approaches. This thesis also underscores the potential for ecological traps under chronic predation, especially when behavioural plasti...