The next time you're tempted to call someone a "bird brain," think twice. Some birds may be craftier than you think. Certain crows can make sophisticated tools. And now, it seems, the birds can even fine-tune their designs.

Scientists used to think that humans were the only animals that made and used tools. But in recent decades, chimpanzees, orangutans, Galapagos woodpecker finches, and other animals have been shown to have such skills.

In the mid-1990s, New Zealand researcher Gavin Hunt found that New Caledonian crows snip twigs and leaves to make tools for digging food out of crevices. The birds use the long, spiky leaves of the pandanus tree to make three kinds of tools with different shapes—wide, narrow, or tapered with a ragged edge.

After analyzing some 5,550 leafy tools, Hunt and his colleague Russell Gray noticed a pattern. In one area, crows used all three kinds of tools. In a surrounding zone, the birds used two types. And in the biggest area, they used only one type, which was also the one that would work well for all sorts of crevices.

This suggests, the researchers say, that crows invented a basic tool design, then improved on it over generations. "As far as we know," Hunt says, "it's the first report in a species other than humans of cumulative change of tool design."—E. Sohn