WHAT THE EVIDENCE SAYS
Researchers analyzed approximately 17 million eBird observations spanning hundreds of North American cities. Winter native bird-richness gradients across latitude differed between urban and nearby nonurban landscapes; introduced birds showed weaker latitude effects, and some urban relationships changed direction under colder conditions.
WHY IT MATTERS / OUR READ
Treat a city as a seasonally shifting ecological landscape rather than a single point on a greener-is-better scale. Migratory dynamics, nonnative assemblages, heat, feeders and neighborhood vegetation can each alter what a city survey actually detects.
CAVEATS / UNCERTAINTY
eBird is opportunistically sampled, so participation and effort bias remain important. Associations do not prove urban refuges are improving population survival or that all cities share the same pattern; model and reporting choices influence city comparisons.
SOURCE
Nature Cities · North American city bird-diversity analysis
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