Body size evolution is generally framed by the benefits of being large, while costs are largely overlooked. An important putative cost of being large is the need to extend development periods, which should increase exposure to predation and potentially select against larger size. In birds, this selection pressure can be important because predation is the main source of offspring mortality and predators should more readily detect the larger nests associated with larger body sizes. Here, we show for diverse passerine birds across the world that, counter to expectations, larger species suffer lower daily nest predation rates than smaller species. This pattern is consistent despite latitudinal variation in predation and does not seem to reflect a tendency of larger species to use more protected nests or less exposed nest locations. Evidence instead suggests that larger species attack a wider array of predator sizes, which could reduce predation rates at nests of large-bodied species. Regardless of the mechanism, the lower daily nest predation rates of larger species yield slightly lower predation rates over the entire development period compared to smaller species. These results highlight the importance of behavior as a mechanism to alter selection pressures, and have implications for body size evolution.

Species traits dataset. Dataset containing the list of the 509 populations and 321 species of passerine birds included in the article, with the corresponding information of life-history, morphological, ecological, behavioral traits and population-level daily predation rates. The source from which information on predation rates was extracted is also specified in the dataset.

Attacking behavior dataset. Dataset containing body sizes of mammal and bird predators that prey species can successfully attack.