Invasion success and tolerance to urbanization in birds.
Cities are considered hotspots of biological invasions, yet it remains unclear why non-indigenous species are so successful in environments that most local native species do not tolerate. Here, we explore the intriguing possibility that humans may be unintentionally introducing species preadapted to persist in such environments. Combining data on historical introductions with information of avian assemblages along urban-wildland gradients, we found that avian species that in their native range proliferate in human-altered environments have been more likely to be transported and introduced to new locations than species confined to the wildland. We also found that such urban dwellers had higher chances to become established because they already had adaptations to cope with novel environments. These findings suggest that the pathway of introduction selects for species preadapted to persist in novel environments, providing an explanation for why non-indigenous birds are so successful in cities. Because the tendency to introduce species associated with human-altered environments continues, there is an urgent need to develop new regulations to prevent future introductions.