Predation risk is associated with head-size-related divergence in breeding phenology in a female sea duck.

Breeding phenological responses to changing environments affect demographics and population persistence. However, inter-individual variation in adjustment of reproductive timing to predation risk has been overlooked. We thus tested whether increasing natural predation risk enhanced or homogenized reproductive timing of females differing in risk sensitivity. Using behavioral (flight initiation distance, FID) and cognitive (relative head volume) proxies of risk sensitivity, we analyzed laying dates of female common eiders (Somateria mollissima) breeding in SW Finland (Baltic Sea) relative to conspecifics under fluctuating predation pressure, while accounting for breeding experience and body condition. We found that high predation risk was associated with a divergence of female eider breeding phenologies depending on cognitive but not behavioral proxies of risk sensitivity. Accordingly, relatively large-headed females, assumed to be more risk-sensitive, bred later than small-headed ones following years of high adult or nest predation risk-plausibly reflecting extended nest prospecting in risk-sensitive individuals-leading to greater laying asynchrony. In contrast, females displaying longer FIDs (more risk-sensitive) bred earlier than those displaying shorter FIDs, but this response was irrespective of the level of predation risk. Because FID was measured late in incubation, we hypothesize that it may more strongly reflect state-dependent parental investment in the current breeding attempt than risk sensitivity during nest initiation, with higher parental investment promoting delayed escape. Additionally, experienced breeders bred earlier than inexperienced ones. More attention should be given to cognitive traits and behaviors associated with risk sensitivity to better understand variation in individual breeding phenology and synchrony.