Bird brains are dense—with neurons

What do you think they're using all those neurons for?
Flickr user Teddy Llovet

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?

We should have expected that answer to be yes. It turns out that flying animals tend to reduce the size of their genomes compared to their non-flying kin. This is the case for both bats and birds. One consequence of this smaller genome is that the cells that carry these genomes end up smaller as well. That tendency has been used to argue that the group of dinosaurs that evolved into birds had already been experiencing a shrinking genome for millions of years beforehand.
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But we could just as easily have applied that logic to neurons. If birds' cells are smaller, more cells can be squeezed into the same volume. Under those circumstances, a small brain wouldn't be as much of a liability as it appears. But logic only gets you so far, so a team of researchers set out to try to count all the neurons in the brains of a range of birds, mostly from the songbirds, corvids, and parrots.

Small songbirds, which weigh as little as 4.5g, really do have small brains. Their brains can weigh as little as a third of a gram and only contain about 100 million neurons. But the heavier birds can have brains that weigh more than a dozen grams and pack in more than 2 billion neurons. On average, birds have twice as many neurons per unit mass as mammals do. So a bird called the goldcrest, which Wikipedia introduces as "a very small passerine bird," weighs a bit more than 10 percent of your average mouse but has more than double the neurons.

The largest parrot brains, by contrast, weigh in at 20g, even though parrot body mass is similar to the heaviest songbirds. The parrot brain also has more than 3 billion neurons. In fact, when it comes to the largest corvids and parrots, the authors write that "their total numbers of neurons are comparable to those of small monkeys or much larger ungulates."

Those cells also have an interesting distribution: as more cells are added, they're preferentially added to a region of the brain called the pallium, which in humans handles things like spatial reasoning, language, and memory. As a result, this area has an impressive number of cells. Ravens and keas (a type of parrot that does live in the fjords) have more neurons in the pallium than a Capuchin monkey. A macaw has more than a rhesus monkey.

As with size and weight, there's no simple relationship between the number of neurons in a brain and its capabilities. But the work certainly presents an argument that we shouldn't assume the thought processes of birds have to be limited by their brains' size. And the authors even suggest there might be some advantages; with more neurons packed closer together, signals shouldn't typically have to travel as far before reaching their destination. Thus, birds might perform information processing a bit more quickly than mammals.