Scientists have long been baffled by the smarts displayed by some birds with tiny brains.

But a new explanation may turn the term "bird brain" on its head: Birds have more densely packed neurons in their brains than other animals, contributing to cognitive ability on par with that of primates, researchers said on Monday.

A macaw's brain may be the size of a shelled walnut, far smaller than that of a macaque monkey -- which has a brain the size of a lemon -- but the parrot has many more neurons, or brain nerve cells, in its forebrain, a region crucial for intelligence, according to a study published in the Proceedings of the National Academy of Sciences.

The researchers were the first to systematically measure neurons in the brains of 20 bird species ranging in size from the tiny finch to the six-foot (1.8-meter) emu.

"For a long time having a 'bird brain' was considered to be a bad thing," said senior author Suzana Herculano-Houzel, a neuroscientist at Vanderbilt University. "Now it turns out that it should be a compliment."

Parrots and crows have cognitive abilities similar to those of primates, the study found.

The birds can make tools and use them to obtain food and solve other problems. They can also recognize themselves in mirrors and plan for future needs, cognitive capabilities only primates were previously thought to have enjoyed.

That is possible probably because the neurons in birds' brains are smaller and more densely packed than those in mammalian brains, the researchers said.

"We found that birds, especially songbirds and parrots, have surprisingly large numbers of neurons in their pallium: the part of the brain that corresponds to the cerebral cortex, which supports higher cognition functions such as planning for the future or finding patterns," Herculano-Houzel said. 

"That explains why they exhibit levels of cognition at least as complex as primates."

The brains of parrots and songbirds contain twice as many neurons as those of primates of the same size and two to four times more than those of rats and mice.

Although the link between intelligence and the number of neurons has not been firmly established, the scientists believe that the brains of birds with the same number of neurons as much larger mammal brains can potentially produce higher "cognitive power" per pound.