Large-brained birds lay smaller but heavier clutches

The brain is among the most energetically costly organs in vertebrates, and thus trade-offs have been hypothesized to exert constraints on brain size evolution. The energy trade-off hypothesis (ETH) predicts that reducing the energy consumption of reproduction or other costly tissues should compensate for the cost of a large brain. Egg production in birds requires a large proportion of the total energy budget, and a clutch mass in some bird species can outweigh the body mass of the female. To date, this hypothesis has mainly been tested in mammals and ectothermic animals such as anurans and fishes. We collated data on adult brain size, body mass and egg-production traits such as clutch size, egg mass and annual broods from published studies, and conducted a phylogenetic comparative test of the interplay between egg-production investment and brain size evolution across bird species. After controlling for phylogenetic relationships and body size, we find a negative correlation between brain size and clutch size across 1395 species, which favored ETH. However, when egg mass was integrated in models, positive associations were detected between brain size and mass of eggs (via egg mass, clutch mass and annual total egg mass). Our results suggest that brain size trades off against egg-production only via certain aspects (e.g., clutch size). By contrast, a positive relationship between brain size and total egg reproduction (e.g., clutch mass and annual total egg mass) implied increased total energy budget outweighing energy allocation across bird species. Our study shows that there is no general energy trade-off between brain size and egg-reproduction investment, and suggests that brain size evolution follows mixed strategies across bird species.