Habitat resource overlap in two broad-ranged sympatric Neotropical forest eagles

Quantifying resource partitioning between co-occurring species has important ecological and evolutionary implications. Yet, few studies compare resource overlap in both geographic and environmental space. We test 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 geographic range. Using landcover and topographic covariates, we quantified resource overlap first using higher resolution (30 arc-sec data) generalized linear models (GLMs), and second using coarser-grain (2.5 arc-min data) environmental ordination. The distribution of both eagles was largely explained by canopy species richness and structural complexity with evergreen forest, but with differing responses to landcover and topography, particularly with the harpy eagle more likely in areas of dense evergreen forest. Both eagles were negatively associated with mosaic forest, with this relationship stronger for the crested eagle. Harpy eagle distribution was restricted by higher elevation and terrain roughness, compared to the crested eagle, whose distribution was more restricted by canopy species richness and structure. From the GLMs, resource overlap was > 92 % in geographical space but reduced to 64 % in environmental space. From ordination, resource overlap was 76 % in environmental space, with randomization tests supporting equivalent environmental space for both eagles. Our results suggest that at the biogeographical scale, crested and harpy eagles share environmental space, but there may be subtle differences in fine-scale habitat preference. We recommend habitat resource overlap be assessed in both geographical and environmental space at multiple resolutions to capture the inherent variability in environmental conditions available to co-occurring species.