The coastal zone provides foraging opportunities for insular populations of terrestrial mammals, allowing for expanded habitat use, increased dietary breadth, and locally higher population densities. We examined the use of sandy beach resources by the threatened island fox (Urocyon littoralis) on the California Channel Islands using scat analysis, surveys of potential prey, beach habitat attributes, and stable isotope analysis. Consumption of beach invertebrates, primarily intertidal talitrid amphipods (Megalorchestia spp.) by island fox varied with abundance of these prey across sites. Distance-based linear modeling revealed that abundance of giant kelp (Macrocystis pyrifera) wrack, rather than beach physical attributes, explained the largest amount of variation in talitrid amphipod abundance and biomass across beaches. δ13C and δ15N values of fox whisker (vibrissae) segments suggested individualism in diet, with generally low δ13C and δ15N values of some foxes consistent with specializing on primarily terrestrial foods, contrasting with the higher isotope values of other individuals that suggested a sustained use of sandy beach resources, the importance of which varied over time. Abundant allochthonous marine resources on beaches, including inputs of giant kelp, may expand habitat use and diet breadth of the island fox, increasing population resilience during declines in terrestrial resources associated with climate variability and long-term climate change.

The island fox (Urocyon littoralis) is native to 6 of the 8 Channel Islands of California, USA. The species experienced a population decline in the 1990s but recovered after predatory golden eagles (Aquila chrysaetos) were relocated and feral pigs (Sus scrofa), a main food source for the eagles, were removed. As part of an ongoing conservation program, the National Park Service conducts yearly health surveys on foxes residing on Santa Rosa and San Miguel islands. In this study, we document non‐invasive measures of stress and nutritional status from fecal samples collected during surveys from 2009 to 2015. We collected samples defecated in traps overnight or during handling and measured concentrations of glucocorticoid (GC) and triiodothyronine (T3) metabolites using validated assays. We used generalized linear mixed models to assess the relationships between hormones, season, island, age class, sex, body condition, reproductive status, and ectoparasite presence. Overall, males had marginally lower fecal T3 concentrations than females. Concentrations of both hormones positively correlated with body condition. Fecal GC production varied seasonally; concentrations were highest from December to February and declined through the summer and fall. During summer, younger females and those with signs of recent reproduction had higher fecal GC concentrations than older females or those without evidence of reproduction. Fecal T3 concentrations did not vary in relation to season, age, or reproductive status, but on San Miguel Island were positively correlated with ectoparasite presence. There were no other significant differences between islands. Our results provide hormone data for island foxes and demonstrate that production varies in relation to seasonal and biological factors. These reference data will serve as a comparison for future health surveys and allow managers to identify factors associated with increased stress or reduced nutritional state. © 2019 The Wildlife Society.