Proceedings of the National Academy of Sciences · 7 July 2026 · From issue 03
Northern swallow populations shifted nesting earliest but still lost the most birds.
Conor C. Taff, J. Ryan Shipley et al., Divergent population trajectories despite similar response to temperature in a widespread aerial insectivore, Proceedings of the National Academy of Sciences.
Tree swallows across North America all respond to warm weather in the same way. Every population lays eggs about one day earlier for each degree the temperature rises. But northern birds are hit harder by climate change than southern birds. Northern populations have shifted their nesting earlier the most, since they warmed up the most. Even so, they still feel strong pressure to nest earlier, especially in warm years. These northern groups have also lost the most birds over time.
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