Duetting, in which two or more individuals call in coordinated unison, is common in birds, yet most research investigating the function of avian duets has focused only on oscine passerines. However, we have discovered that duetting occurs in 61 species (59%) in the family Rallidae (rails), one of the highest known rates in any bird clade, and that rail duets are also significantly associated with year-round territoriality and habitat type. We applied a comparative approach to study the occurrence of duetting relative to socio-ecological traits and call properties in 103 rail species, with the prediction that duetting rails produce low-frequency calls that spread well over short distances in densely vegetated habitats. Using a model correcting for phylogenetic signal, we found that duetting is the ancestral state in Rallidae, and that duetting rails show a trending effect for being sedentary and non-migratory. Contrary to our predictions, there was no significant correlation between rail duetting and social bond length, breeding system, breeding latitude from the equator, or sexual dimorphism, as year-round territoriality and forest or heterogeneous habitats were the strongest predictors of duets. Despite the prevalence of duetting in Rallidae, few studies have tested duet functions such as territory defense in this family, and our comparative phylogenetic study lays the groundwork for future research, as little remains known about the behavioral ecology and vocal interactions of many rails.

The data in this dataset include measurements obtained from sound recordings downloaded from the Macaulay Library of the Cornell Lab of Ornithology (http://macaulaylibrary.org/) and from Xeno-Canto (http://xenocanto.org/) that were processed in Raven Pro Version 1.5.1 (Center for Conservation Bioacoustics 2014). In the related Zenodo Software dataset are one NEXUS and one TRE file of 103 species in Rallidae that we generated in FigTree Version 1.4.2 (https://github.com/rambaut/figtree), and three R scripts that we generated and processed in R Studio Version 1.4.1 (R Core Team 2020).

R Studio Version 1.4.1 or newer is required to open the R scripts in the Zenodo Software dataset, and FigTree Version 1.4.2 is required to visualize the NEXUS file (RallidFigTree) and TRE file (RallidTree) from Zenodo. Microsoft Excel is sufficient to open the comma-separated (CSV) files in the Dryad dataset.