Association of social group with both life-history traits and brain size in cooperatively breeding birds

Social group is associated with life-history traits and can predict brain size variation in cooperative primates and some other mammal groups, but such explicit relationships remain enigmatic in cooperatively breeding birds. Indeed, some compositions of social group in cooperative species (e.g., helper number and group size) would affect the fitness of breeders by providing alloparental care. Here, we conducted comparative tests of the relationship between the social group and both life-history traits and brain size across 197 species of cooperatively breeding birds using phylogenetically controlled comparative analyses. We did not find any correlations between helper numbers and both life-history traits and brain size. However, we found that maximum group size was positively associated with clutch size. Moreover, average group size has positive associations with body mass and relative brain size. Our findings suggest that helper numbers cannot promote variation in relative brain size, while larger groups may predict bigger brains in cooperatively breeding birds.