Investigations into niche dynamics and interspecific interactions are crucial to assess the vulnerability of threatened species to competition and environmental change. The Whenua Hou Diving Petrel Pelecanoides whenuahouensis is a critically endangered seabird, specializing in breeding in sand dunes in Aotearoa (New Zealand). The species is at risk from competition for burrow sites with the abundant, habitat‐generalist Common Diving Petrel Pelecanoides urinatrix chathamensis , among other terrestrial and near‐shore threats (storm‐induced erosion and vessel‐based light pollution). However, Whenua Hou Diving Petrel foraging niches at sea and associated threats are poorly understood. We sampled blood and feathers from both species on Whenua Hou (Codfish Island) during 2017–19 and used stable isotope (δ 15 N and δ 13 C) analyses to quantify foraging niche dynamics and interspecific interactions. During incubation periods, when birds are restricted by central place foraging, diving petrels exhibited clear niche segregation. Whenua Hou Diving Petrels were more generalist feeders with wider isotopic niches than Common Diving Petrels, which exhibited restricted isotopic niches and fed in neritic waters. During non‐breeding periods, foraging niches expanded, and segregation was reduced, as both species dispersed into the Southern Ocean. These patterns were largely consistent among years, despite varying climatic conditions. However, Common Diving Petrel foraging niches appeared to shrink during El Niño conditions, suggesting environmental sensitivity. In contrast, the consistently stable, generalist foraging niches of Whenua Hou Diving Petrels suggested potential limited sensitivity to competition and impacts of environmental change at sea. Recovery efforts should, therefore, focus on addressing the previously identified terrestrial and near‐shore threats.