Heterogeneity, Productivity, and Migration Drive Continental Evenness Patterns of Bird Assemblages

ABSTRACT Aim Evenness quantifies similarities in abundances among species in an assemblage and may influence processes such as species coexistence and the supply of ecosystem services. Previous work has failed to identify generalised patterns of how productivity and habitat heterogeneity influence evenness and moreover has not considered seasonal variation in evenness arising from migration. Therefore, our goal was to quantify the interplay of productivity, habitat heterogeneity, and migration on evenness patterns of bird assemblages. Location Contiguous United States. Time Period Contemporary. Group Birds (613 species). Methods Using relative abundance maps from eBird, we computed evenness of bird assemblages for 27 × 27 km grid cells at weekly temporal resolution. We used generalized linear mixed‐effects models to evaluate the influences of productivity (normalized difference vegetation index), habitat heterogeneity (Shannon diversity of land cover), and the dominance of migratory species on evenness. Results Productivity and heterogeneity interacted to influence evenness such that there was a positive productivity–evenness relationship in high‐heterogeneity landscapes but no relationship in low‐heterogeneity landscapes. Evenness was highest during the pre‐breeding and breeding seasons when migratory taxa were present, but a high dominance of migrants reduced evenness and generally dampened evenness–productivity relationships. Main Conclusions Evenness increased with productivity in high‐heterogeneity—but not low‐heterogeneity—landscapes, indicating that hypotheses relating to energy availability and niche partitioning should be considered jointly. Higher evenness during the growing season likely reflects the presence of low‐abundance migrant taxa, whereas the negative effects of migrant dominance on evenness are likely driven by one or few high‐abundance species. The dampening effect of migrant dominance on evenness–productivity relationships in many situations supports the notion that migrants concentrate in high‐productivity landscapes.