Birds are smart. They use tools, engage in social learning, plan for the future, and do a variety of other things that were once thought to be exclusively the stuff of primates. But hundreds of millions of years of evolution separate mammals and birds, and structurally, their brains look very distinct. Plus there's the whole size thing. If you look at a bird's head, it's clear that there's not a whole lot of space for mental hardware in it. So how do the birds manage with smaller brains?
While other studies have tackled a lot of the structural differences, a new one released this week in PNAS shows that, to some extent, size doesn't matter. Its authors show that birds pack neurons into their brains at densities well above densities in mammals' brains, putting some relatively compact bird brains into the same realm as those of primates when it comes to total cell counts.
And the funny thing is, we probably should have known this was the case.
If you look at a typical avian brain without knowing much about brains, you'll mostly be impressed by the size (or lack of it). Some of the heaviest brains in birds are found in the macaws, and those weigh in at under 25 grams. The raven, a large bird with a well-deserved reputation for intelligence, has a brain that is typically around 15g. That's in the same neighborhood as a rabbit.
If you know your way around some neuroanatomy, however, other things will stand out. Many of the structures we associate with higher cognition in mammals (and especially in primates) either aren't clearly there or look rather different in birds, which suggests that bird cognition has to be radically different from the cognition in mammals.
But as we have identified the proteins that act as key regulators of mammalian brain development, we have discovered that the same proteins are all there in birds, too. Tracking their expression as the brain develops has allowed us to determine that some of the brain structures that look physically different in birds and mammals actually have the same developmental history and express the same suite of genes when mature. Finally, manipulating the activity of these genes affects bird and mammalian brains in similar ways.
So, all the same basic pieces seem to be there in both birds and mammals, which leaves the issue of raw horsepower. Mammalian brains are simply so much bigger that it seems inevitable that they could get more done.
But size isn't everything. Neural capabilities seem to be based on the number of neurons present, as well as the number of connections they can establish. Could birds simply cram more neurons into the same amount of physical space and thus get more done with a smaller brain?