Elevation gradients shape morphology and acoustic phenotype across isolated songbird populations

Javier Oñate Casado , Juan Carlos Illera, Tereza Petrusková, Adam Petrusek, Henrik Brumm

Elevational gradients provide natural experiments for examining how environmental variation shapes animal phenotypes. In birds, geographic variation in body size may also influence song frequency because vocal production is constrained by morphology. We tested whether elevation and temperature are associated with coordinated variation in morphology and song frequency in Berthelot’s pipit (Anthus berthelotii), a songbird endemic to Macaronesia. We measured song characteristics and male morphology across elevational and thermal gradients on three islands: Tenerife, La Palma, and Madeira. We tested relationships between environmental conditions, song frequency, a composite body-size index, and individual morphological traits using linear mixed-effects models. Song peak frequency decreased by approximately 249 Hz per 1000 m increase in elevation and by 570 Hz per 10 °C decrease in annual mean temperature. These relationships were consistent among islands. Body size showed similar elevational and thermal responses: the multivariate body-size score increased by 0.316 units per 1000 m of elevation, and by 0.690 units per 10 °C decrease in temperature. Higher, colder environments were therefore associated with both larger males and lower-frequency songs. This parallel variation is consistent with predictions from Bergmann’s rule and suggests that environmental effects on morphology translate into geographic variation in song frequency. Thus, environmental gradients can structure multiple, functionally connected phenotypic traits within a species. By linking elevational variation in climate with coordinated changes in morphology and song across geographically isolated populations, this study shows how ecological variation can propagate across different levels of phenotype and thereby contribute to geographic variation in animal behavior.