A recent study illustrated that Zebra finches can recognize the song or call of at least 50 members of their flock. They use this ability to find a lost member or to call and see if it is safe to return to the nest.
They need this ability because they usually travel in colonies of 50 to 100 birds and they split up and then come back together. They have distance calls that they use to identify where they are and to find members of the flock.
While it has been known that songbirds are capable of communicating sounds with complex meanings, the latest research shows that songbird brains are capable of complex vocal communication. This also reflects on their high level of intelligence. Imagine how smart other animals are.