Supplementary material from "Living in stable social groups is associated with reduced brain size in woodpeckers ( Picidae )"

Instead, stable pair-bonding and bi-parental care have been identified as correlates of larger brains in birds. We investigated the relationship between brain size and social system within the family Picidae , using phylogenetically controlled regression analysis.

We found no specific effect of duration or strength of pair bonds, but brain sizes were systematically smaller in species living in long-lasting social groups of larger sizes. Group living may only present a cognitive challenge in groups in which members have individually competitive relationships; we therefore propose that groups functioning for cooperative benefit may allow disinvestment in expensive brain tissue.