Of all analysed species traits, only those directly linked to parental care and offspring provisioning showed strong relationships with brain size. Our data showed species that fed their young for a longer time were species with some of the biggest brains (again, relative to body size).
The development style mattered a lot, too. Birds can be easily divided into two large groups. Precocial species are those where juveniles hatch from eggs already relatively well developed (such as chickens, ducks, geese), requiring little to no feeding.
Altricial birds, in contrast, hatch severely underdeveloped. Usually their hatchlings are blind, naked and fully dependent on their parents’ care. This group includes some of the best-known bird groups we encounter every day, such as sparrows, tits, robins and finches.
Because altricial birds receive relatively more care from their parents, we predicted that they should also be able to evolve bigger brains – a pattern that we see clearly in our data.
It also seems mammalian brain size is indeed constrained by the amount of energy mothers can transfer to their offspring until weaning. When it comes to having a large brain, it appears parental love and care come before any subsequent learning.
(ed: the importance of mothering in the evolution of the human species should not be underestimated.