Species-energy relationships (SER) have been used scarcely to predict responses of biodiversity to changes in land use. In this work, two hypothesis of the SER were tested on bird communities along urbanization gradients: the increasing population size (IPS) and the niche position (NP) hypotheses. The IPS hypothesis states that sites with more primary productivity have more individuals, decreasing the probability of extinction and increasing species richness. The HPN predicts that sites with greater primary productivity and environmental heterogeneity allow the availability of a greater variety of resources and the presence of specialist species, thus increasing species richness. The hypothesis of habitat heterogeneity was evaluated by which more complex habitats provide more variety of resources and greater species richness. Birds were counted along three urban-rural gradients in the Pampean region of Argentina during the breeding season. The productive energy was measured from the NDVI and the environmental heterogeneity through the Shannon-Wiener diversity index, calculated with the percentage coverage of different substrates. Bird richness and habitat diversity were higher at intermediate levels of urbanization, while primary productivity had a negative relationship with urbanization. Although there was a positive relationship between bird richness and abundance, there was no positive relationship between bird abundance and primary productivity. Bird richness showed a positive relationship with habitat diversity and primary productivity; although path analysis revealed that the habitat diversity increased bird richness through an increase of bird abundance. The NP hypothesis was the most consistent to predict the bird richness along urban-rural gradients.

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.