Baldwin, Justin W.García Porta, JoanBotero, Carlos A.2025-06-262025-06-262023-07-15Baldwin, J.W., Garcia-Porta, J. & Botero, C.A. Complementarity in Allen’s and Bergmann’s rules among birds. Nat Commun 14, 4240 (2023). https://doi.org/10.1038/s41467-023-39954-92041-172310.1038/s41467-023-39954-9https://hdl.handle.net/20.500.14352/121883Thanks to Jonathan Losos for comments on an early draft. C.A.B. was funded by NSF, award number DEB 184147. J.G.-P. was supported by a Beatriu de Pinós fellowship from the Generalitat de Catalunya (2020 BP 00147) and by the program “Atracción de Talento Investigador Modalidad I” from the Comunidad de Madrid (2022-T1/AMB-24171).Biologists have long noted that endotherms tend to have larger bodies (Bergmann’s rule) and shorter appendages (Allen’s rule) in colder environments. Nevertheless, many taxonomic groups appear not to conform to these ‘rules’, and general explanations for these frequent exceptions are currently lacking. Here we note that by combining complementary changes in body and extremity size, lineages could theoretically respond to thermal gradients with smaller changes in either trait than those predicted by either Bergmann’s or Allen’s rule alone. To test this idea, we leverage geographic, ecological, phylogenetic, and morphological data on 6,974 non-migratory terrestrial bird species, and show that stronger family-wide changes in bill size over thermal gradients are correlated with more muted changes in body size. Additionally, we show that most bird families exhibit weak but appropriately directed changes in both traits, supporting the notion of complementarity in Bergmann’s and Allen’s rules. Finally, we show that the few families that exhibit significant gradients in either bill or body size, tend to be more speciose, widely distributed, or ecologically constrained. Our findings validate Bergmann’s and Allen’s logic and remind us that body and bill size are simply convenient proxies for their true quantity of interest: the surface-to-volume ratio.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/journal articlehttps://doi.org/10.1038/s41467-023-39954-9https://www.nature.com/articles/s41467-023-39954-9open access598.2591.1591.5575.8Animal physiologyEvolutionEvolutionary ecologyMacroecologyZoologíaAvesFisiología animal (Biología)Ecología (Biología)Evolución2401 Biología Animal (Zoología)2401.20 Ornitología2401.13 Fisiología Animal2401.06 Ecología Animal