A parrot has grasped the concept of zero, something humans can't do until at least the toddler phase, researchers say.

Alex, a 28-year-old African gray parrot who lives in a lab at Brandeis University in Massachusetts, has a brain the size of a walnut. But when confronted with no items on a tray where usually there are some, he says "none."

A 2003 study in the journal Nature, for example, found that common marsh birds called coots can recognize and count their own eggs, even when other eggs are in the nest.

Black-capped chickadees were recently found to warn colleagues of danger by chirping about the size and actual threat of individual predators. The language of prairie dogs includes a word for humans.

Some animal intelligence is hauntingly familiar, like the male monkeys that pay to see female monkey bottoms. And studies show that monkeys, dogs and rats all know how to laugh.