Many species of animals can recognize each other's voices. For example, if a horse sees another horse it knows walk by, and then hears that horse's whinny, it won't be surprised, but if it then hears a different horse, it will look up and try to figure out what's going on. Many species go beyond this and have "signature contact calls" -- specific patterns of sound they make to identify themselves, the equivalent of saying "I'm Bob! I'm Bob!" And their species-mates can recognize that as their tag.
This new study, by Cornell's Karl Berg, has found that the green-rumped parrotlet goes much farther than this. Not only do individual parrots have these calls, but they can use each other's call signs to identify each other: not merely the equivalent of saying "I'm Bob! I'm Bob!" but saying "Hey Ted! Come here!" And on top of that, these signs aren't innate: they appear to be learned around an age of 3 weeks, and similar to the names of their parents.
Which is to say, these are more than call signs: these are bona-fide names, used by parrots in much the same way we use names ourselves.