Why Are Some Birds So Smart? Their Brains Are Neurally Dense

Little by little, the well-established myth of the stupid bird is crumbling. Just a few months ago, researchers reported that some birds are as smart as apes. Now another team of scientists say that bird brains pack way more brain cells per square inch than those of primates and other mammals. They published their findings in the Proceedings of the National Academy of Sciences.

The brains of humans and apes include a structure called the neocortex, which houses some of our most sophisticated thought processes and mental tasks. Birds don’t have neocortices at all, and so for years, scientists assumed that meant they were unintelligent. As it turns out, we were just being self-centered. Birds don’t need neocortices; a similar structure called a pallium does their mental heavy lifting.

One bird stereotype is accurate: Compared to, say, a human or a chimpanzee, birds do have little brains. Yet “corvids and some parrots are capable of cognitive feats comparable to those of great apes,” the authors write. “How do birds achieve impressive cognitive prowess with walnut-sized brains?”

To find out, the researchers examined the brains of 28 different bird species, from tiny zebra finches to towering emus. They examined the birds’ brain tissue at a cellular level, measuring the number and distribution of neurons in each region. They compared the neural density to that found in the brains of mice, rats, primates, and artiodactyls (even-toed ungulates).

As it turns out, when it comes to brain cell density, birds have them all beat. The pallium was crammed wall-to-wall with brain cells. Parrot and songbird brains contained twice as many neurons as similar-sized primate brains, and two to four times as many neurons as those of rodents. Birds also dedicate way more brainpower to their pallia than we do to our neocortices. The pallium houses 33 to 55 percent of a songbird’s brain cells, and 46 to 61 percent of a parrot’s. By comparison, the human neocortex hosts only 19 percent of our brain cells.

Senior author Suzana Herculano-Houzel, a neuroscientist at Vanderbilt University, said in a press statement: “In designing brains, nature has two parameters it can play with: the size and number of neurons and the distribution of neurons across different brain centers, and in birds we find that nature has used both of them.”

These findings open a new pathway in understanding how brains evolve. Generally speaking, in order to advance, brains have had to get bigger, and bigger brains are linked to more cognitive capacity. But, Herculano-Houzel said, “bird brains show that there are other ways to add neurons: keep most neurons small and locally connected and only allow a small percentage to grow large enough to make the longer connections."

But no evolutionary advantage is truly free. Does a denser bird brain require more energy to run than ours, which are spacious by comparison? If so, where is that energy coming from?

“Something I love about science is that when you answer one question, it raises a number of new questions,” said Herculano-Houzel.