A major component of human-induced global change is the deliberate or accidental translocation of species from their native ranges to alien environments1,2, where they may cause substantial environmental and economic damage3,4. Thus we need to understand why some introductions succeed while others fail. Successful introductions tend to be concentrated in certain regions2, especially islands and the temperate zone, suggesting that species-rich mainland and tropical locations are harder to invade because of greater biotic resistance1,5-9. However, this pattern could also reflect variation in the suitability of the abiotic environment at introduction locations for the species introduced3,9-11, coupled with known confounding effects of nonrandom selection of species and locations for introduction8,12-14. Here, we test these alternative hypotheses using a global data set of historical bird introductions, employing a statistical framework that accounts for differences among species and regions in terms of introduction success. By removing these confounding effects, we show that the pattern of avian introduction success is not consistent with the biotic resistance hypothesis. Instead, success depends on the suitability of the abiotic environment for the exotic species at the introduction site.