Endotherms show smaller extremities (Allen’s rule) and larger body sizes (Bergmann’s rule) in colder climates, thereby reducing heat loss. Coloration can also be important in thermoregulation, as darker coloration absorbs more radiant energy than lighter coloration (Bogert’s rule or thermal melanism hypothesis). Extremities lacking or bearing thinner covers of feathers or fur —such as ears, legs and bills— can be important thermal windows to exchange heat. Yet, no study to date has simultaneously investigated the coloration and morphology of body extremities in relation to temperature. In 566 species of Furnariida (Neotropical ovenbirds and allies), we tested with phylogenetic comparative methods whether body extremity coloration, morphology and plumage coloration jointly respond to temperature. We predicted that, in colder climates, bills and tarsi would be darker and smaller as well as associated with darker plumage and larger body sizes. Consistent with this prediction, we found that bills are darker, smaller and are associated with darker plumage and larger body sizes in colder climates. Tarsi are darker and associated with darker plumage and larger body sizes in colder climates, but tarsus coloration is not associated with tarsus length, which, contrary to predictions of Allen’s rule, was longer in colder climates. Using phylogenetic path analyses, we found that climate affects body extremity coloration both directly and indirectly through its effects on habitat type, plumage coloration and morphology. Our study suggests that temperature promotes an integrated phenotypic response of coloration and morphology across body extremities, plumage and body size.

Coloration of bills, tarsi and plumage was extracted from illustrations of species of Furnariida (Neotropical ovenbirds, antbirds and allies) available at the Birds of the World (https://birdsoftheworld.org/) using the Color Histogram plugin (Prodanov et al., 2010) in ImageJ–Fiji (Schindelin et al., 2012). Additionally, plumage color data for comparison with colors taken from illustrations was obtained from museum specimens.

Data on phylogenetic relationships of species, climate, habitat type and morphological traits was obtained from published literature (Harvey et al., 2020; Tobias et al., 2022).

References

Harvey, M.G., Bravo, G. A., Claramunt, S., Cuervo, A. M., Derryberry, G., Battilana, J., Seeholzer, G. F., Shearer, McKay J., O’Meara, B. C., Faircloth, B.C., Edwards, S.V., Pérez-Emán, J., Moyle, R. G., Sheldon, F. H., Aleixo, A., Smith, B. T., Chesser, R. T., Silveira, L.F., Cracraft, J., Brumfield, R. T., and Derryberry, E. P. (2020). The evolution of a tropical biodiversity hotspot. Science, 370:1343–1348.

Prodanov, D., Roussel, S., Trekker, J., Dresselaers, T., and Himmelreich, U. (2010). ImageJ in biomedical imaging applications, in A. Jahnen and C. Moll (eds), Proceedings of the 3rd ImageJ User and Developer Conference, pp. 20 – 27.

Schindelin, J., Arganda-Carreras, I., Frise, E., Kaynig, V., Longair, M., Pietzsch, T., … Cardona, A. (2012). Fiji: an open-source platform for biological-image analysis. Nature Methods, 9:676–682.

Tobias, J. A., et al. (2022). AVONET: morphological, ecological and geographical data for all birds. Ecology Letters, 25:581-597.