Abstract
Coexistence among sympatric, functionally similar species often hinges on niche differentiation, especially as resource competition intensifies during the breeding season. Individual specialisation (IS) can promote coexistence by narrowing individual niches or increasing divergence among individuals. In colonial seabirds, aggregation at limited breeding sites and central-place foraging amplify both intra- and interspecific competition. Here, we assess seasonal shifts in individual and population isotopic niche widths in two sympatric cormorant species to elucidate the mechanisms underlying their coexistence. We analysed isotopic composition (δ (13)C and δ (15)N) in multiple-tissues to produce repeated measures within 111 individuals of red-legged cormorant (Poikilocarbo gaimardi) and imperial shag (Leucocarbo atriceps) captured on the Pirén Islet (Los Lagos, Chile) during breeding and non-breeding seasons. Multivariate generalised linear mixed models estimated isotopic niche components: total niche width (TNW), within-individual component (WIC), and between-individual component (BIC). We estimated IS (i.e., the extent to which individuals exploit a narrower subset of the population niche) as BIC/TNW. L. atriceps exhibited 2.2-fold greater TNW than P. gaimardi during non-breeding and 2-fold greater during breeding. IS differed markedly between species: L. atriceps showed a higher IS during non-breeding (0.541 vs 0.213 in P. gaimardi), but decreased by 79.3% during breeding, whereas P. gaimardi increased IS by 52.1%. Niche width overlap was asymmetric and seasonally variable: P. gaimardi exhibited high overlap with L. atriceps (95.7% non-breeding, 89.6% breeding), whilst L. atriceps showed lower overlap (48.3% non-breeding, 43.7% breeding). Competition indices increased substantially during breeding in both species (305% in L. atriceps, 221% in P. gaimardi). Results suggest that coexistence relies on multiple mechanisms, including subtle population niche differentiation, contrasting IS between species, and divergent resource-use strategies. The high niche width overlap and narrower niche of P. gaimardi suggest greater competitive vulnerability for this Near Threatened species. Conservation of foraging habitat heterogeneity and prey availability is crucial for maintaining ecological opportunities that sustain these coexistence mechanisms.