Environmental variability has long been postulated as a major selective force in the evolution of large brains. To test this hypothesis, we assembled information of brain size for over 1,200 bird species and used remote-sensing analyses to estimate temporal variation in plant productivity. As expected, larger brains (relative to body size) are more likely to occur in species exposed to larger environmental variation throughout their geographic range. Reconstructions of evolutionary trajectories are consistent with the hypothesis that larger brains (relative to body size) have evolved when the species invaded more seasonal regions, although the alternative - that the species already possessed larger brains when invaded more seasonal regions - cannot be ruled out.