Stable isotope analyses of archival specimens have revealed trophic declines over the past 150 yr in a growing number of predatory pelagic seabirds that breed in the Hawaiian Islands. However, they have not examined whether isotopic shifts occurred primarily during a specific phase of the annual cycle, which could allow us to better identify the causes of trophic declines and potential consequences for affected populations. We evaluated seasonal (breeding versus nonbreeding season) foraging habits of 2 ecologically distinct species, Newell's shearwater Puffinus newelli and Laysan albatross Phoebastria immutabilis, and extended this analysis back 50 and 100 yr, respectively. Our assessment relied on amino acid delta N-15 proxies for nutrient regime use (delta N-15(Phe)) and trophic position (Delta delta N-15(Glu-Phe)) from 2 tissues (feather and bone collagen) reflecting different time scales. Both study species exhibited season-specific isotopic shifts resulting in more pronounced seasonality in modern populations. We also identified inter-species differences in nutrient regime use, regardless of season. Laysan albatross experienced a trophic decline exclusive to the breeding season, while their nonbreeding season foraging ecology has remained constant over the past century. In contrast, the nutrient regime at the base of the food chain for Newell's shearwaters during the nonbreeding season underwent a shift within the last 50 yr, and the trophic decline they experienced was heavily weighted toward the nonbreeding season. Efforts to mitigate potential fitness consequences of future trophic declines might benefit from focusing on fisheries management near the Hawaiian Islands, where susceptible seabirds forage during winter/spring, rather than the entire North Pacific.
