Abstract
Biodiversity is unevenly distributed along elevational gradients. The predominant hypothesis is that macroevolutionary dynamics and climatic niche conservatism explain today's elevational patterns of biodiversity, but the alternative energy efficiency hypothesis emphasizes modern ecological interactions related to energy budgets. We test these competing hypotheses by examining seasonal elevational ranges for 10,998 bird populations in 34 mountain regions. Multiple lines of evidence support the energy efficiency hypothesis, including that many mountain birds do not seasonally track their thermal niche with high fidelity while simulation models based on optimal energy balancing under current environmental conditions yield predictions that tightly match empirical data. Our results reveal that altitudinal migration, which is widespread yet considerably understudied, is a behavioral mechanism fulfilling the same ecological function as long-distance latitudinal migration. Overall, this work provides a better understanding and predictive capacity for mountain birds under global change.