Structural and socioeconomic features of cities predict migratory bird species richness

Submitted by Michi on 22 March 2024.

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Cities are complex environments shaped by the decisions that humans make. Our policies, city planning decisions and every day activities result in different city structures and socioeconomic realities that then determine what food and nesting resources are available for wildlife.

Birds are common visitors and inhabitants of cities, using city parks and people’s backyards to refuel during migration and to nest in during the breeding season, but our knowledge of the kinds of migratory birds that we find in different types of cities is still limited, particularly when studied across many cities.

In our study we investigated whether the structural and socioeconomic features of cities can predict the species richness of migratory birds that would generally select different habitats during the breeding season. We compiled a dataset from publicly available open-source data - eBird for the bird data and US human census data to characterise cities.  

The final dataset had data from over 300 bird species across thousands of cities in the United States, allowing for robust inferences. We modelled the relationship between features of cities (housing density, median income, city age, and commuting time), environmental disturbance (measured by the human footprint index) and species richness by fitting generalized linear models to data.

    Migratory city birds: House finch and Eurasian collared-dove on a fence in Galt, California.

Our results show that the features of cities can predict the species richness of migratory birds and that overall, species are responding to city variation in surprisingly similar ways.

One clear finding was that human commuting time in a city was positively related to avian biodiversity, with the former representing the extent of urban sprawl.

Additionally, although we had expected cities with more environmental disturbance to have lower species richness, cities seem able to support some migratory bird species even in more disturbed areas and may act as refuges to birds. This seemed particularly evident when the surrounding region had high levels of human-caused disturbance, like agricultural fields and industry.

      Cliff swallows nesting under a bridge in Davis, California.

It is possible that cities in these regions may have relatively high vegetation indices compared to the surrounding region. As many migratory birds are currently facing declines and fragmentation of their preferred natural breeding habitats mainly due to land cover change, knowledge of the preferences and ability of avian migrants to tolerate human-caused disturbance in different cities is of high importance.