Endothermic vertebrates (birds and mammals) show much larger relative brain sizes than ectothermic ones. The gap is widely attributed to the evolution of endothermy itself, which makes sustained high body temperatures possible. However, other variables that emerged or proliferated after endothermy arose, in particular greater and more extended parental provisioning, no doubt also contribute to this gap. Here, we conducted a targeted comparison of lizards and precocial birds to estimate the relative contribution of these factors. Both taxa share large amniotic eggs, lack post-hatching provisioning, and have overlapping body temperatures. After statistically removing additional confounding variables, we found that the statistical effect of endothermy amounts to a near-tripling of brain size, and thus accounts for about half of the difference in brain size between lizards and birds in general. A separate analysis revealed that the effect of extended parental provisioning in birds is associated with a near-doubling of brain size. These results suggest that various processes that were enabled or facilitated by endothermy, in particular parental provisioning after hatching or birth, played an equally important role in the evolution of increased brain size among endotherms.