Multi-scale habitat overlap in two broad-ranged sympatric Neotropical forest eagles reveals shared environmental space and habitat use

Quantifying resource partitioning between co-occurring species can provide insight into processes facilitating coexistence by closely related species, a fundamental question in ecology. We tested whether the habitat requirements of two closely related Neotropical forest eagles, the Crested Eagle Morphnus guianensis and Harpy Eagle Harpia harpyja, differ at fine and coarse resolutions across their shared geographical range. Using landcover and topographical covariates, we quantified potential resource overlap first using higher resolution (30 arc-s, similar to 1-km(2) data) generalized linear models (GLMs), and secondly using coarser-grain (2.5 arc-min, similar to 4.5-km(2) data) environmental ordination to capture the potential effect of scale on habitat overlap. The distribution of both eagles was largely explained by canopy tree species richness and canopy structural complexity, with peak suitability of 60-80% evergreen forest cover. Both eagles were negatively associated with mosaic forest and cultivated areas. From the GLMs, habitat overlap was >93% in geographical space but was reduced to 73% when considering environmental space, a proxy for resource overlap. From ordination (principal component analysis), resource overlap was 67% in environmental space, with randomization tests supporting equivalent environmental space for both eagles. Our results suggest that at the continental scale, Crested and Harpy Eagles share identical environmental space when quantified at fine and broad scales, with little difference in distribution and habitat use. At the continental scale used here, both eagles can coexist, presumably with sufficient habitat heterogeneity for coexistence when they occur in close proximity. Therefore, further research is required at the local level to capture fully where coexistence at the local scale is facilitated more by fine-scale habitat selection, or difference in diet between two species with indistinguishable habitat use.