High-elevation birds grow more slowly but to heavier weights than low-elevation birds

Documenting how growth strategies diverge among populations or species occupying different geographical locations can contribute to understanding life-history evolution. However, the existing literature on the geography of growth focuses on latitudinal gradients, leaving the growth-elevation relationship enigmatic. The knowledge gap limits our ability to capture a full picture of growth evolution at macroecological scales because organisms typically experience different selective pressures across these two geographical gradients. We filled this gap using data from 163 populations of 111 passerine species breeding in mainland China over an elevational span of 8-4500 m. Phylogenetic comparative analyses revealed that controlling for potentially confounding variables, the growth rate of nestling body mass decreased and the nestling period became longer as elevation increased. Also, high-elevation nestlings fledged at a heavier body mass relative to the adult than their low-elevation congeners. The observed growth patterns, coupled with fewer, larger eggs and prolonged parental care as demonstrated by earlier studies, suggest that avian life histories have slowed down toward high elevations. The life-history strategy, which stands in contrast to the fast pace of life toward high latitudes, is likely a response to increasingly stressful conditions with elevation, especially the limited availability of food and oxygen.