Thirty years ago, biologists began to predict how climate change would alter species’ ranges, extinctions, and their roles in ecosystems (1). Since then, evidence has begun to confirm these nascent predictions. The current global temperature rise of ∼1 °C has shifted species up mountains and latitudes (2), altered the timing of key life events such as flowering and migration (3), and eradicated populations and species (4). As the climate continues to change, these impacts on biodiversity are expected to accelerate (5). In PNAS, Freeman et al. (6) add to the growing drumbeat of climate change’s pervasive impacts on nature by demonstrating the mountaintop extirpations of five tropical bird populations. In 1985, tropical biologists charted an elevational transect in the mountains of Cerro de Pantiacolla, which rise from the Amazonian lowlands in Peru. Along this transect, they recorded each bird they encountered. Three decades later, biologists returned to resurvey the original transect. Such historical resurveys offer a particularly effective way to gauge biotic changes in response to long-term environmental responses, assuming they apply similar methods and effort. Oftentimes, these comparisons are difficult to ascribe to climate change because other factors, such as land use, also change during this time. In this study, the study area remained undisturbed, making it an exceptional long-term catalog of nature’s history. What did change was temperature, which rose 0.4 °C between surveys. After comparing observations between 1985 and 2017, the biologists discovered that the upper elevational ranges of birds had shifted upslope an average of 68 m, which was consistent with the temperature rise. Mountaintop species suffered the most. High-elevation species lost on average 110 m of elevational range and substantial habitat … [↵][1]1Email: mark.urban{at}uconn.edu. [1]: #xref-corresp-1-1