Previous studies have shown the brains of songbirds and parrots contain very large numbers of neurons in their forebrains, sometimes even more than monkeys.
But while the idea that cognitive performance is linked to an animal's total number of neurons seems intuitive, it lacks sufficient evidence.
A recent comparison of apes, corvids, and pigeons found that total neuron numbers are a poor predictor of absolute cognitive power, although they might account for an animal's speed of learning or its adaptability to situations.
Other studies disagree. Their findings suggest that the total number of neurons in a specific part of the bird forebrain, known as the pallium, does matter regarding memory, learning, reasoning, and problem-solving.
But what numbers should we be counting? The relative number of neurons, or the absolute? In other words, is being big-bodied enough to make a bird smart? Or is it all about how big their brain is in proportion to their body?
A team of researchers now think it's a bit of both, a previously overlooked compromise.
First, the authors estimated the numbers of neurons present in the palliums of 111 bird species. Then, they compared these numbers to over 4,400 innovative ways for birds to use food or feed.
Ultimately, they found bird species with higher numbers of neurons in their pallium were also more likely to be innovative.
However, when there were more neurons in the pallium, there tended to be fewer elsewhere in the brain.
"If a bird contains a disproportionately large number of neurons in a big chunk of their forebrain, then this should produce a brain that is both larger in absolute terms and relative to its body size," the authors conclude.
The findings suggest that bird intelligence depends on a "disproportionate allocation of neurons to cognitive tasks".
As such, the authors argue that their findings support the hypothesis that intelligence depends on both the total number of neurons and the way those neurons connect different brain areas.
In short, measuring intelligence is much more complicated than counting neurons or determining how densely packed those cells are in a given space.
Instead, the authors think bird intelligence has to do with how widely distributed networks in the brain are controlled and integrated.
"This last finding agrees with the notion that animals that have large brains merely because they have very big bodies are not necessarily the most intelligent," the authors write.
When researchers compared the development of all the bird species in their study, they found those birds that develop more slowly as fledglings tend to have larger numbers of neurons in their pallium.
This suggests greater bird intelligence may require an extra phase of brain development after an egg hatches, during which time the pallium is scaled up.
"The number of time fledglings spend in the nest as their brains develop might also play a crucial role in the evolution of intelligence," says biologist Louis Lefebvre from McGill University in Canada.
"Larger species of crows and parrots, known for their intelligence, spend longer in the nest, which allows more time for the brain to grow and accumulate pallial neurons."
Recent brain research also suggests that human intelligence is superior to other primate intelligence not because of our brain's size but because our brains are more flexible or possess greater synergy.
The same could be true among birds, too.
What makes some animal species smarter than others remains an open question for scientists. So until we know more, perhaps it is wise to stop ridiculing bird brains merely for their size.
The study was published in Nature Ecology and Evolution.