Same species, different risks: predation shapes hormonal profiles in a breeding sea duck, the common eider

Prey are predicted to respond to humans as they would to predators. Hence, human disturbance can elicit costly physiological and behavioural antipredator responses. However, human presence can also shelter prey from human-averse predators (the human 'scarecrow effect'). Whether this 'scarecrow effect' reduces perceived predation risk and promotes prey reproductive investment, as predicted by life-history theory, remains unclear. To test this, we monitored complementary hormonal markers in two geographically proximate colonies of common eiders Somateria mollissima breeding under contrasting conditions: one exposed to low human disturbance but high predation risk, and the other to high human disturbance (intense tourism) shielding birds from predators (i.e. low predation risk). We measured proxies of energy expenditure [baseline corticosterone (CORT)] and thyroid hormone levels [triiodothyronine (T3) and thyroxine (T4)], acute stress response (stress-induced CORT levels) and parental effort [baseline prolactin (PRL) levels]. Hormonal profiles differed markedly between colonies. Eiders in the low-disturbance/high-predation colony displayed higher baseline CORT and T3 levels than those in the high-disturbance/low-predation one, suggesting higher energy expenditure-possibly reflecting a higher energetic cost of incubating under elevated predation pressure. Stress-induced CORT levels were similar across colonies, suggesting that eiders maintain sensitivity to acute stressors despite a human 'scarecrow effect.' Supporting the hypothesis of increased parental effort under reduced perceived predation risk, incubating females displayed higher baseline PRL levels in the high-disturbance/low-predation colony. Altogether, these results are consistent with a human 'scarecrow effect' that reduces perceived predation risk without diminishing sensitivity to acute stressors, and that is associated with increased reproductive investment. Our research highlights the value of integrating multiple hormonal markers to understand wildlife responses to human disturbance and predation risk.