Coastal marine ecosystems worldwide have undergone such profound transformations from over-fishing that trophic interactions observed today might be artifacts of these changes. We determined whether the trophic level of an endangered seabird, the Marbled Murrelet (Brachyramphus marmoratus), has declined over the past 100 years after the collapse of Pacific sardine (Sardinops sadax) fisheries in the late 1940s and the recent declines of similar fisheries in central California. We compared stable-isotope signatures of delta15N and delta13C in feathers of museum specimens collected before fisheries decline with values in murrelet feathers collected recently. Values of delta15N in prebreeding diets declined significantly, 1.4 per thousand or 38% of a trophic level, over the past century during cool ocean conditions and by 0.5 per thousand during warm conditions, whereas postbreeding values of delta15N were nearly constant. The delta13C values in prebreeding diets declined by 0.8 per thousand, suggesting an increased importance of krill in modern compared with historic prebreeding diets, but postbreeding diets did not change. Stable-isotope mixing models indicated that the proportion of energetically superior high-trophic-level prey declined strongly whereas energetically poor low-trophic-level and midtrophic-level prey increased in the prebreeding diet in cool years when murrelet reproduction was likely to be high. Decreased prey resources have caused murrelets to fish further down on the food web, appear partly responsible for poor murrelet reproduction, and may have contributed to its listing under the U.S. Endangered Species Act.