Climate variation facilitates differences in community assemblages and dynamics (e.g., parasitism, predation), which may have cascading effects on foraging ecology and immunocompetence. Consequently, diet and disease prevalence are likely to vary among populations in species with broad geographic ranges. However, patterns of community assemblage can be complicated to predict across small spatial scales, and the impacts of differing assemblages on widely distributed species are not well understood. Song sparrows (Melospiza melodia), a North American passerine, exhibit extensive phenotypic variation across their broad range that has been found to correlate with climate. Here, we tested whether stark climate differences among isolated Channel Island song sparrow (Melospiza melodia graminea) populations lead to variation in foraging niche and infection rates. Specifically, we (1) used stable isotope analysis (n = 90) to infer foraging niche width among three populations (San Miguel, Santa Rosa, and Santa Cruz Islands), and (2) inferred infection rates between our two windiest and most humid islands (San Miguel and Santa Rosa Islands) using blood cell counts as a proxy (n = 47). We found δ13C levels were elevated on San Miguel Island, suggesting that San Miguel Island song sparrows forage on greater amounts of CAM and C4 plants and/or on plants growing in high salinity soil. Additionally, we found elevated δ15N levels on Santa Rosa Island, providing evidence for Santa Rosa Island sparrows foraging at a higher trophic level (i.e., more invertebrates, less vegetation). However, we did not find differences in either white blood cell or abnormal red blood cell counts between Santa Rosa Island and San Miguel Island birds. Collectively, our findings suggest that climate has complex effects on organismal biology among the islands, and diet and infection rates may not vary consistently with predictions of the theory of island biogeography. As habitat restoration efforts move forward, our study provides valuable insights regarding the management of this California Species of Special Concern.