Tropical cities lose the greatest share of bird diversity
A global analysis of more than 10,000 species finds that urbanisation weakens one of nature's most persistent geographical patterns.
Tropical cities lose a substantially greater share of their bird diversity than cities at high latitudes, according to a global analysis of observations covering more than 10,000 species.
The research was published in the Proceedings of the National Academy of Sciences in August, but its authors presented the findings for a wider audience on 7 November. They report that the familiar rise in species richness from the poles towards the equator is three times weaker in urban areas than in natural landscapes.
Across the world, cities held about 32% fewer bird species than comparable natural areas. The difference was strongly dependent on latitude. A one-kilometre square close to the equator had, on average, 42% fewer species when urban rather than natural, compared with a 12% reduction near 65 degrees latitude.
"Our analyses show the power of community-driven science."
Jory Griffith and study co-authors
The team divided the world into one-by-one-kilometre cells and classified each as urban, suburban or natural using human population density and land-use data. Bird occurrence came from eBird, with cells retained only where reporting was sufficient to capture most of the local species pool. The researchers also ran stricter data-quality tests and examined whether cities had been built in unusually species-poor places.
One explanation for the tropical effect lies in specialisation. Tropical bird communities contain a high proportion of species tied to particular habitats or resources, while cities tend to favour adaptable generalists. Those specialists were disproportionately absent from urban cells. The result is not simply fewer birds, but a levelling of the geographical pattern that normally makes tropical ecosystems exceptionally diverse.
Greener cities supported more species at a given latitude than cities with less green space. That association suggests that retaining native vegetation, wetlands and connected habitat can reduce losses, although the global analysis does not show that any single design will work equally well in every climate or bird community.
The study measures species richness rather than population size, breeding success or ecological function. It also relies on community-science records, which are unevenly distributed despite the authors' sampling thresholds and sensitivity tests. A species recorded in a city may be scarce or transient, while an absent record does not always mean true absence.
The authors are now examining whether changes in urban bird communities alter interactions such as pollination and predation. Their present results show that urbanisation does not have a geographically uniform effect: development near the equator removes a larger fraction of the local bird community precisely where natural diversity is greatest.
10 Nov 2026
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