Data from: 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 years (2003-2023), 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 anti-predator 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.