The role of size assortment in structuring neotropical bird communities

This study tested confamilial size assortment at three different latitudes, representing a gradient of productivity and stability: the northern subtropics (Rio Grande Valley), the equatorial zone (Amazonian Peru) and the austral subtropics (Paraguayan Chaco). Size assortment is the likely diminished persistence of a species by presence of morphologically similar species; temporally synchronous and spatially sympatric species competing for similar resources should exhibit distinct characters in ecomorphological space, molded over time to reduce the chance of competition. Despite least intensive sampling effort at the Amazon site, it is the most speciose (238 species, 78 common) compared to the Chaco (147, 76) and Rio Grande (61, 24) sites. Size assortment was tested by comparing mean mandibular measurements of confamilials in a real pool against those in a null pool. The pattern of size assortment was pervasive in 68% of the 22 families tested, with most being animal consumers or omnivores, represented by a high percentage of insectivores.