Teaching isn’t often seen in animals other than humans—and it’s even more difficult to demonstrate in animals living in the wild rather than in a laboratory setting. But researchers studying the Australian superb fairy-wren (Malurus cyaneus) in the wild think the small songbirds (a male is shown in the photo above) practice the behavior. They regard a female fairy-wren sitting on her nest and incubating her eggs as the teacher, and her embryonic chicks as her pupils. She must teach her unhatched chicks a password—a call they will use after emerging to solicit food from their parents; the better they learn the password, the more they will be fed. Since 1992, there’s been a well-accepted definition of teaching that consists of three criteria. First, the teacher must modify his or her behavior in the presence of a naive individual—which the birds do; the mothers increase their teaching (that is, the rate at which they make the call) when their chicks are in a late stage of incubation. Second, there must be a benefit to the pupil, which there clearly is. Scientists reported online yesterday in Behavioral Ecology that the fairy-wrens also pass the third criteria: There must be a cost to the teacher. And for the small birds, there can be a hefty price to pay. The more often a female repeats the password, the more likely she is to attract a parasitical cuckoo, which will sneak in and lay its eggs in her nest. From careful field observations, the scientists discovered that at nests that were parasitized, the females had recited their password 20 times an hour. But at nests that were not parasitized, the females had called only 10 times per hour. Superb fairy-wrens thus join a short but growing list of animal-teachers, such as rock ants, meerkats, and pied babblers.