Seabird moult timing and duration: Implications for at-sea threat exposure worldwide

Tubenosed seabirds (order procellariforms) are the most threatened bird group. Threats generally act on two distinct elements of their life-history, either breeding at terrestrial colonies, or during the nonbreeding season while at sea foraging and migrating. Moult is often overlooked as another critical life-history component that can determine their vulnerability and exposure to threats at sea. Using 13,092 geo-referenced citizen-science photographs from publicly available and private collections, we scored primary feather moult for 6 gadfly petrel species ( Pterodroma spp.) as a representative and understudied group of tubenosed seabirds. To investigate environmental conditions of areas where gadfly petrels moult, we quantified moult phenology for 1336 individuals. Moult activity overlapped with breeding and/or migration in all species, a surprising result, as these processes are temporally offset in many other birds. Sea surface temperature predicted moult activity in most species, with the probability of active moult being greater in areas with higher sea surface temperatures. Higher sea surface temperatures are associated with lower marine productivity, so gadfly petrels undergo moult in less productive waters than they occupy at other times. This possibly reflects avoidance of adverse conditions that disproportionately affect gadfly petrels with compromised plumage. Our findings indicate that moult is important for understanding tubenosed seabird distributions, and affects exposure to substantial threats. Knowledge of how moult informs threat exposure can refine best-practice in conservation planning for seabirds, by informing key management tools such as designating marine protected areas.