In the tropical mountains that are Earth’s most biodiverse terrestrial ecosystems, climate change has set in motion an “escalator to extinction” as species move up to cooler elevations, occupy shrinking ranges, and then go extinct. Yet we do not know how many species are threatened, nor the mechanisms that explain why warming temperatures lead some species but not others to shift to higher elevations. My research directly addresses these puzzles, combining (1) The Mountain Bird Network, a global monitoring network to rapidly diagnose tropical species’ vulnerability, with (2) Tech Mountain, a detailed field study to test alternative mechanisms that could drive the patterns uncovered by the Mountain Bird Network. Packard Foundation support will enable me to discover how and why tropical montane species are responding to climate change, with fundamental implications for the future of Earth’s most biodiverse terrestrial ecosystems.