Species diversity typically increases from higher to lower latitudes, but the regional-scale variation along this geographic gradient remains unclear. It has been suggested that species diversity throughout Amazonia generally increases westward toward the Andes, but this pattern and its environmental determinants require further investigation for most taxa. Using mist-net data on understorey birds, we evaluated patterns of species richness using two approaches by addressing methodological issues that influence local species richness and the determinants of species richness across Pan-Amazonia. Specifically, we examined (i) the disparity between observed and expected species richness obtained from geographic range maps; (ii) how species' eco-morphological traits influence their detection and relative abundance; (iii) the spatial variation in estimated local species richness after controlling for sampling effort; and (iv) the environmental determinants of estimated richness. We found no evidence for a longitudinal westward increase in estimated species richness, but there was a marked difference between the northern and southern banks of the Amazon river. Species detection and abundance were modestly explained by species traits, and estimated richness was weakly associated with latitude, aboveground biomass and climatic aridity. We found that observed variation in the local species richness was primarily driven by differences in sampling effort, while estimated species richness showed modest variation across large spatial scales and was poorly explained by environmental and spatial gradients. Despite wide variation in local species richness, we conclude that at broader scales, species richness of understorey bird assemblages was surprisingly stable across Pan-Amazonia, suggesting that evolutionary processes may be important in determining these patterns at larger scales.