Calling someone a birdbrain is usually meant as an insult. But many birds are quite clever. Take, for example, the mockingbird.
To you and me, one mockingbird might look pretty much like any other. But from the birds' perspective, the same isn't true of humans. Mockingbirds can tell one person from another with great skill, especially when that person poses a threat.
In one experiment, scientists had volunteers approached and touched mockingbird nests once a day for four days. By the second day, the birds began their warning calls and attack runs sooner than they had the day before.
By the fourth day, the birds appeared to be able to spot the human intruders as soon as they began to approach the nests. On the fifth, day, however, a new person approached and touched the birds' nests. And the birds were slower to react.
In other words, the birds weren't simply protecting their nests with the same ferocity no matter who approached. They quickly recognized repeat intruders and were quick to target them. Because the new intruder wasn't familiar, the birds took longer to size him up.
Mockingbirds might not be the only bird species to distinguish between people. Other species that have adapted to human environments might be good people spotters, too. In fact, this ability might be part of what allows some birds to thrive in urban environments.