Big cities with small green areas hold a lower species richness and proportion of migrant birds: A global analysis

Several studies around the world have shown that species richness and proportion of migrant species in bird communities increase toward the poles as a result of increased climatic seasonality and a considerable annual variation of resources. Since migrant species can use urban green areas (UGAs), they may be negatively affected by the loss of green areas and human disturbances. The aim of this study was to analyze the global pattern of migrant species richness and proportion in UGAs, considering the effects of both climatic seasonality and urbanization. Data of bird communities in UGAs were gathered through a search of scientific articles, book chapters, thesis works and unpublished data. 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 total bird species richness and migratory species richness and proportion in each dataset to environmental and methodological variables that controlled for different sampling efforts among studies. A total of 37 cities from four continents were analyzed. Total bird species richness increased with the size of UGAs and in the Northern Hemisphere. As expected, species richness and proportion of migrants increased with increasing annual range of temperature and precipitation, and was higher in the Northern Hemisphere. Species richness and proportion of migrants decreased with increasing population size of cities but increased with size of UGAs surveyed. The results suggest that big cities have a negative impact on richness and proportion of migrant species. However, increasing the size and number of UGAs may benefit migrant species presence in cities.

Automated summary

Studies have shown that bird species richness and migrant species proportion increase towards the poles due to climatic seasonality, with big cities having a negative impact on migrant species. However, increasing the size and number of urban green areas may benefit migrant species presence in cities.