Australian birds track climate warming over decades via shifts in bill morphology.

Global warming is widely predicted to drive adaptive responses in species' morphology, yet such responses have rarely been demonstrated to track changing climate through time. We investigated associations between avian bill size and changes in seasonal climate extremes using museum specimens of 57 Australian passerine species (Meliphagides). We employed location-specific climate data from the 4119 sites where specimens were collected to test whether change in the frequencies of summer (≥35 °C) and winter (<5°C) extremes over the preceding 20 years of each specimen's collection date was associated with bill size variation as predicted by thermoregulatory hypotheses. Rapid warming in summer and winter was associated with bill size variation in accordance with the bill's role in heat exchange. Our study provides critical evidence that bill size is tracking climate change through time in response to changing thermoregulatory demands. A critical future question is whether such changes facilitate species persistence.