Can animals count? People count easily, from the time we are little kids and learn our one-two-threes. But what about other species? Careful observations in the wild support the idea that some can.

The American coot is a duck-like North American bird. Sometimes a coot will try and sneak one of its eggs into a neighbor's nest. Some coots recognize the deception, and roll the stranger's egg out again. Others don't catch on, and raise the stranger's egg as their own.

Researcher Bruce Lyon at the University of California, Santa Cruz, and his team observed a series of coot nests for four seasons, noting which birds could spot a stranger's eggs and which couldn't. Then they compared how many eggs each kind laid.

Their finding? The coots that couldn't tell when a stranger's egg had been stuck into their nest laid that many fewer of their own--sneak one in, the coot lays one fewer.

Sneak two in, the coot lays two fewer. Coots that successfully spotted intruders knocked them out again and then laid the same number they normally would.

If the number of eggs laid were independent of how many the coot sees in its nest, coots that allow a stranger's egg should wind up with one egg extra. But they don't.

Lyon argues that, along with egg recognition, the coots must therefore be doing some version of "I see I have nine eggs here--time to stop laying now." And that means coots count.

Other researchers aren't convinced. But though the debate goes on, the case for animal counting just got stronger...by the weight of one coot.