Large cities and the loss of green areas exclude migrant birds: a global analysis

Several studies around the world have shown that the proportion of migrant species in bird communities increases toward the poles as a result of greater climatic seasonality and a considerable annual variation of resources. In this context, urban areas may impose a barrier to bird migrants given their buffered seasonality of resources and human disturbance. The aim of this study is to analyze the global pattern of migrant species proportion in urban green areas, considering the effects of climatic seasonality as well as the effects of urbanization. Data of bird communities in urban green areas were gathered through a search of scientific articles, book chapters, and thesis. Datasets that included a list of observed species, the numbers of parks surveyed and other methodological characteristics were considered for the analysis. Then, generalized linear models were used to relate the proportion of migratory species in each dataset to environmental and methodological variables that controlled for different sampling effort among studies. A total of 32 cities from four continents were analyzed. As expected, the migrant proportion increased with the annual range of temperature and precipitation and was higher in the Northern Hemisphere. However, the proportion of migrants decreased with the population size of cities but increased in those datasets with the highest maximum size of green areas surveyed. Although the global pattern of migrant proportion in urban green areas follows a similar pattern than those found in natural areas, the results obtained suggest that urbanization have a negative impact on this global pattern by reducing the proportion of migrant species in large cities. Moreover, green area loss in cities may impact negatively the proportion of migrant species.