Navigating danger: how environmental cues and individual traits shape breeding dispersal in the endangered common eider

Breeding dispersal, or movement between successive breeding sites, is insufficiently understood. A “win-stay, lose-switch” (WSLS) strategy may facilitate predator avoidance, but this strategy may be modulated by individual traits. We studied the drivers of breeding dispersal within and between breeding islands in female common eiders (Somateria mollissima), a ground-nesting site-fidelic sea duck, in the northern Baltic Sea. Rapidly increasing predation led us to expect increased dispersal over time. Using data spanning 21 yr (2003 to 2023), we investigated which individual traits (nest success, age, body condition, boldness, relative head size) and environmental factors (predation risk, breeding density) influenced breeding dispersal. Within-island breeding dispersal distances showed moderate repeatability (r = 0.45), indicating individually consistent dispersal distances across years, but dropped to 0.20 after accounting for significant fixed effects, highlighting the roles of environmental and individual context in dispersal decisions. Consistent with the WSLS strategy, dispersal distances within islands and the likelihood of switching islands increased following predator-induced breeding failure and years with high island-specific nest predation risk. Older females switched islands less frequently, likely due to accumulated experience and age-dependent antipredator strategies. Island switching was also more likely in years with abundant white-tailed sea eagles (Haliaeetus albicilla), the main predator of adult eiders, and directed toward larger islands offering tree cover, indicating risk perception beyond the immediate nest surroundings. Although philopatry can increase vulnerability to predation, flexible dispersal strategies may facilitate population persistence in this endangered population under intense predation. Future studies should assess the fitness outcomes of dispersal across diverse predation landscapes.