Why are exotic birds so successful in urbanized environments?

Many nonindigenous organisms, including birds, are often restricted to human-altered environments within the region of introduction. The classical explanation is that human-related alterations make the environment easier to invade by reducing biotic resistance and offering new niche opportunities. However, the pattern may also reflect that many more species have been introduced in humanaltered environments and/or that traits associated with invasion success and the ability to thrive in these environments are related. In this chapter, we argue that if we want to fully understand why exotic organisms are mainly successful in human-altered environments, we need to see the invasion process as a set of stages with different probabilities of being transited. Applied to birds, this framework suggests that there is a high probability that an exotic species ends up associated with human-altered environments if the species: (1) is more abundant (and hence more available for introduction) in urbanized environments; (2) has a higher chance to be successfully transported, as it is already habituated to humans; and (3) has a higher probability to be introduced in an urbanized environment, where most humans live. If these arguments are true, then the exotic species is likely to successfully establish itself in the new region because the species should already have the traits needed to persist in the novel environment. Although more supporting evidence is needed, the proposed framework provides a general solution for the paradox that many invaders are more successful in the new environment than most native species.