Omega-3 enriched chick diet reduced the foraging areas of breeders in two closely related shearwaters from contrasting marine environments.

Seabirds have evolved several life-history characteristics to help buffer environmental stochasticity. However, particularly during the breeding season, seabirds may be affected by reductions in prey availability and localised oceanographic conditions caused by variations in the environment. The increase in sea surface temperature, triggered by accelerated global warming, is impairing phytoplankton production of omega-3 fatty acids (FA). Here, we assessed the ecological role of omega-3 FA on chick development and subsequently on breeders' foraging behaviour in two closely related shearwaters species foraging in contrasting marine environments. We supplemented chicks with omega-3 FA pills or with control placebo pills and monitored chick growth, chick health status and breeders' at-sea foraging behaviour using global positioning system (GPS) devices. We found that omega-3 chick supplementation reduced the 95% kernel utilization distribution of Cape Verde shearwaters' short trips, but overall breeders kept a similar foraging pattern between treatments, potentially influenced by predictable prey patches off the West African coast. In contrast, for Cory's shearwaters, the parents of the omega-3 group greatly reduced the foraging effort. This suggests that the proximity to productive prey patches around the colony may help birds to adjust their effort and, therefore, energy expenditure, to changes in the development of their offspring, as driven by their nutritional status. Overall, our results suggest a link between a chick diet enriched in omega-3 FA and parents' foraging effort, providing insight into their ability to cope with a changing and increasingly stochastic marine environment.