Sympatric seabirds: exploring multidimensional niche partitioning among four cormorant species

The competitive exclusion theory suggests that closely related and ecologically similar species living in sympatry should differ along some axes of their n-dimensional ecological niches to reduce interspecific competition. Cormorants (Phalacrocoracidae) are foot-propelled pursuit-diving aquatic birds that constitute an interesting model to study resource partitioning at a local level, due to their colonial breeding habits and limited foraging ranges. The aim of this study was to investigate the partitioning of the multidimensional niche among 4 cormorant species that coexist within a marine inlet along the Argentine Patagonian coast: red-legged, rock, neotropic, and imperial cormorants. For this, we estimated their diet composition, isotopic niches, foraging areas, and reproductive schedules, while assessing pairwise multidimensional overlap. While our analysis showed some degree of pairwise overlap in certain dimensions—mainly spatial and temporal, and more pronounced between rock and neotropic cormorants—the 4 cormorant species overall exhibited resource partitioning. This study highlights that, despite residing and foraging in close proximity in a spatially delimited coastal inlet situated at a latitude marked by strong seasonality, the studied seabirds, belonging to the same guild, clearly partitioned the use of their trophic resources by occupying different volumes, or combinations of dimensions, within the ecological niche. The cumulative effect of the pairwise segregations found in trophic, spatial, and temporal dimensions was the key to determining the overall niche partitioning between these sympatric seabird species.