The fitness benefit of association with humans: elevated success of birds breeding indoors

Many animals have successfully adapted to human proximity, with dramatic increases in abundance as a consequence. Although such transitions imply a fitness advantage, the fitness benefits of associations between animals and humans have not been thoroughly investigated. In a comparative study of nest predation, I compared predation rates in 6874 nests of 11 species of birds with sympatric populations breeding indoors and outdoors. Mean nest predation rates were 23.5% outdoors, but only 1.0% indoors, because corvid nest predators never entered buildings. There was a negative correlation between nest predation rate and the proportion of individuals breeding indoors, implying that as species became more adapted to humans, and hence breeding indoors became more frequent, there was a significant decrease in nesting failure that translated into a difference in reproductive success due to reductions in nest predation. Finally, the difference in predation rate between outdoor and indoor nests was related to time since urbanization and number of generations since urbanization, implying that initially there was a large selection differential followed by reduced fitness differences between birds breeding outdoors and indoors due to gradual adaptation to human proximity by reproducing birds. With a high intensity of natural selection, these findings suggest that such adaptation to human proximity may only take a few hundred generations, as shown by several species that have only recently become associated with humans.