Niche theory predicts that to reduce competition for the same resource, sympatric ecologically similar species should exploit divergent niches and segregate in one or more dimensions. Seasonal variations in environmental conditions and energy requirements can influence the mechanisms and the degree of niche segregation. However, studies have overlooked the multi-dimensional aspect of niche segregation over the whole annual cycle, and key facets of species co-existence still remain ambiguous. The present study provides insights into the niche use and partitioning of two morphologically and ecologically similar seabirds, the common (CDP, Pelecanoides urinatrix ) and the South Georgian diving petrels (SGDP, P. georgicus ). Using phenology, at-sea distribution, diving behavior and isotopic data (during the incubation, chick-rearing and non-breeding periods), we show that the degree of partitioning was highly stage-dependent. During the breeding season, the greater niche segregation during chick-rearing than incubation supported the hypothesis that resource partitioning increases during energetically demanding periods. During the post breeding period, while the observed species-specific latitudinal differences were expected, CDP and SGDP also migrated in very divergent directions. This may indicate the implication of processes other than inter-species niche competition. Our results demonstrate the importance of integrative approaches combining concepts and techniques from different fields to better understand the co-existence of ecologically similar species. The stage-dependent and context-dependent niche segregation highlights the need for whole-year and multiple-site studies of niche partitioning of sympatric species. This is particularly relevant in order to fully understand the short and long-term effects of ongoing environmental changes on species distributions and communities.