A country-wide examination of effects of urbanization on common birds

Urbanization forms one of the most drastic alterations of the environment and poses a major threat to wildlife. The human-induced modifications of the landscape may affect individual’s fitness and thereby result in population declines. Research on how urbanization affects fitness traits has shown mixed results, but typically contrasted data from few urban and non-urban sites collected over short time frames from single species. It thus remains unknown whether we can generalize across species, whereas such knowledge is crucial for population predictions that are needed for conservation management. Here, we use data from a nation-wide citizen science project to examine variation in survival and body mass and size of common passerine birds, collected along an urbanization gradient in the Netherlands over an 8-year period. Although the overall association between urbanization and survival was slightly negative, there was strong support for lower survival in three species, and higher survival in two of the 11 species examined. Effects of urbanization on body mass and size also varied but were far less strong and there was no evidence that they mediated the impacts on survival. Our results imply that body mass and size cannot be used as indicators for urban-associated patterns of survival. Furthermore, the species-specific survival responses indicate that care should be taken when predicting the effects of ongoing urbanization for communities, because even closely related species can show different responses. Moreover, the contrasting survival successes among species suggests that ongoing urbanization may lead to shifts in community structure and loss of biodiversity.