Latitude and communal living are associated with the evolution of joint territory defense in New World songbirds

There is great variation across species in how many individuals participate in territorial defense. Here, we test the hypotheses that joint territorial defense by pairs or family groups is more common in songbird species that (1) nest in tropical latitudes, (2) exhibit weak sexual selection, (3) maintain long-term social bonds, (4) defend year-round territories, (5) nest cooperatively, and (6) are sedentary. We conducted the first broad-scale test of these hypotheses by performing 3,177 playback experiments across the Americas to measure territorial defense behaviors for 264 species. We found support for three of our six predictions: tropical species, cooperative nesters, and species with long-term social bonds are indeed more likely to jointly defend territories, but other variables were unrelated to joint territorial defense. Latitudinal zone was the strongest predictor, suggesting that tropical environments select for joint territory defense above and beyond the life-history traits we included in our analysis. The remaining traits that predicted territorial defense describe aspects of communal living, though the association with long-term social bonds was marginal. Overall, we document a strong latitudinal gradient wherein joint territorial defense is consistently more common in the tropics even when accounting for different life-history traits of tropical birds.