Fig. 1: Global test of Bergmann’s rule across 9962 (99.7%) avian species.

Distribution of log-transformed body mass across species geographic ranges, shown as their geometric centroids (a). Model selection procedure for predicting log body mass (b), with six temperature measures assessed within species geographic ranges, as sole fixed effects; AIC—Akaike Information Criterion, r²—coefficient of determination. An exemplar Bergmann’s model (c), showing decreasing body size with max temperature of all months; see Supplementary Fig. 1 for surrogate models based on the other temperature measures (evaluated in b). The shaded area around the trend line is simple shading to facilitate reading. The p values refer to the significance of temperature effect and whether it differs from zero as derived from a two-tailed test. The results were obtained with phylogenetic linear regression by phylolm models on a single maximum clade credibility phylogenetic tree.