The effects of rivers as barriers for species dispersal can be expected to vary according to both species traits and the characteristics of rivers themselves. In this context, our aim was to investigate dissimilarities in species composition across opposite banks of two major Amazonian rivers, the Juruá and the Tapajós, and relate them to each river's characteristics and history.

The study was conducted in upland forests on opposite banks of two contrasting Amazonian rivers: the Juruá and the Tapajós. The Juruá River, a highly meandering and sediment-rich river, drains the Solimões sedimentary basin. In contrast, the Tapajós River is an entrenched, low-sediment river flowing through the cratonic region of the ancient Brazilian shield. These rivers represent distinct geomorphological and geological settings, allowing us to assess their differential effects on bird species compositions.

Data were collected over multiple time periods, spanning several decades. The earliest surveys were conducted in the 1990s. Additional data were collected in 2012 and 2013, followed by a more recent sampling effort in 2022.

Upland forest bird assemblages, including species with different ecological traits, such as varying body sizes, diets and dispersal abilities, ensure an evaluation of how species characteristics interact with riverine features in shaping bird community structure.

We used standardised bird surveys to assess taxon (genus, species and subspecies) composition of upland bird assemblages on both sides of each river. Occupancy and abundance data were obtained using mist-net lines. Differences in composition were analysed using statistical methods to assess community dissimilarity and test for significant differences between opposite margins. We assessed the relative contributions of river basins, geographical distances and environmental variables in explaining dissimilarities in bird taxon composition across the Juruá and Tapajós rivers.

We found significant differences in bird assemblages between the opposite margins of the stable Tapajós, but not between the margins of the more dynamic Juruá. These findings suggest that the Tapajós, with its entrenched channel and long-term geological stability, acts as a stronger biogeographic barrier than the Juruá, which has a broader floodplain but a more dynamic and meandering course. The less pronounced barrier effect of the Juruá is likely due to its dynamic current and historical geomorphological processes, which facilitate species dispersal across the river and its floodplain.

Our findings are consistent with the notion that geomorphological history and fluviomorphic stability can influence the strength of Amazonian rivers as biogeographic barriers, but broader replication is needed to generalise this pattern across the Amazon basin. Rivers with a more dynamic and shifting course, like the Juruá, appear to exert weaker dispersal limitations, whereas stable rivers, like the Tapajós, create more pronounced differences in species composition. This supports the notion that rivers experiencing a more dynamic history exert a weaker geographic barrier effect, and that geomorphological setting and landscape history are important determinants of the effect of Amazonian rivers on the dispersal and diversification of Amazonian vertebrate taxa.