Thermal Similarity Attenuates the Strength of Bergmanns Rule in Birds

Thermal similarity is the strongest predictor of how strongly Bergmann’s rule holds across 5,715 bird species in 513 genera, weakening body-mass temperature clines when thermal contrast is low.

• Why it matters: Support for Bergmann’s rule in birds varies widely among lineages, and the drivers of this inconsistency are unclear.
• What they did: Using a global comparative dataset of 5,715 bird species across 513 genera, the study tested competing explanations for variation in body mass–temperature relationships, finding thermal similarity as the top predictor.
• The result: In genera with significantly negative body mass–temperature relationships, higher thermal similarity was linked to less negative slopes, with the positive association supported in dense and semi open habitats and among nonmigratory birds, but not clearly in open habitats or among migratory birds.

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