Parental provisioning drives brain size in birds
Larger brains should be adaptive because they support numerous eco- and socio-cognitive benefits, but these benefits explain only a modest part of the interspecific variation in brain size. Notably underexplored are the high energetic costs of developing brains, and thus the possible role of parental provisioning in the evolution of adult brain size. We explore this idea in birds, which show considerable variation in both socio-ecological traits and the energy transfer from parents to offspring. Comparative analyses of 1,176 bird species show that the combination of adult body mass, mode of development at hatching, relative egg mass, and the time spent provisioning the young in combination strongly predict relative brain size across species. Adding adult eco- and socio-cognitive predictors only marginally adds explanatory value. We therefore conclude that parental provisioning enabled bird species to evolve into skill-intensive niches, reducing interspecific competition and consequently promoting survival prospects and population stability. Critically, parental provisioning also explains why precocial bird species have smaller brains than altricial ones. Finally, these results suggest that the cognitive adaptations that provide the behavioral flexibility to improve reproductive success and survival are intrinsically linked to successful parental provisioning. Our findings also suggest that the traditionally assessed cognitive abilities may not predict relative brain size.