Endocrine drivers of incubation performance in a High Arctic seabird, the little auk (Alle alle)

Parental care in long-lived seabirds is mediated by complex hormonal and behavioural mechanisms, yet the links between endocrine profiles and incubation performance remain poorly understood. We investigated how baseline and stress-induced levels of prolactin and corticosterone relate to incubation behaviour in the little auk (Alle alle), a High Arctic seabird with biparental care. Using a standardised capture-restraint protocol, we measured hormonal responses to an acute stressor in males and females as well as their subsequent nest attendance. As expected, acute stress elicited a decrease in prolactin and an increase in corticosterone, consistent with patterns reported for other long-lived polar seabirds. Sex differences were non-significant: females only tended to have higher circulating prolactin and a stronger corticosterone stress response, potentially reflecting that females spend less time at the colony outside incubation, whereas males also invest in territorial defence and social activities. Baseline corticosterone was positively associated with time spent outside the colony in parents that maintained incubation (attentive individuals), suggesting a role in supporting foraging activity, while no such relationship was observed in birds that subsequently abandoned incubation (neglectful individuals). No differences in stress-induced hormone levels were found between attentive and neglectful individuals, raising the possibility that relevant endocrine changes occurred outside the sampling window. These findings highlight the subtle interplay between hormones, sex-specific roles, and parental behaviour in a long-lived seabird, and suggest that both proximate endocrine mechanisms and individual life-history strategies shape incubation decisions.