ABSTRACT: Ecologically similar species partition their use of resources and habitats and thus coexist due to ecological segregation in space, time, or diet. In seabirds, this segregation may differ over the annual cycle or vary inter-annually. We evaluated niche segregation in 3 sympatric storm-petrel species (Hydrobates melania, H. leucorhous, and H. microsoma) from the San Benito Islands, Mexico, during 2012 and 2013. We used diet samples and carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic values obtained from egg membranes, blood, feathers, and prey. We used krill samples to delineate marine δ¹³C and δ¹⁵N isoscapes for the Baja California Peninsula. During the breeding season, storm-petrels segregated regarding diet composition, stable isotope values, and isotopic niches. H. melania consumed higher trophic-position prey from neritic waters, while H. leucorhous and H. microsoma foraged on lower-trophic position prey from oceanic waters. Isotopic niches among species did not overlap in 2013, whereas those of H. microsoma and H. leucorhous overlapped in 2012. The feeding strategies of H. melania varied among breeding phases, and adults consumed different prey items from different areas compared to those of their offspring. H. microsoma adults and their chicks consumed the same prey items but from different habitats. During the non-breeding period, niche segregation between species persisted, except for H. microsoma and H. leucorhous during the molt of primary (P1) and undertail cover feathers. These 3 sympatric species coexist through niche segregation based on prey items and foraging areas that vary seasonally and year-round, probably due to changes in oceanographic conditions and the distribution and availability of prey.