Brood parasitic species such as brown headed cowbird can avoid the cost of parenting through laying eggs in nests of host species such as red-winged blackbirds. While chicks of blackbirds can easily recognise members of their species through mirroring their parents, cowbird chicks can’t. They are born in nests of hosts, copying whom would mean they are unable to recognize their own species. One possible way that chicks can deal with this problem is through having an innate preference for calls that are unique to their species. However, it is not known what parts of the brain helps them with this recognition problem.
In the first experiment, cowbird chicks and adults were either exposed to the password call or dove coos. In cowbirds of both ages, greater activity was observed in the brain region associated with song familiarity in response to the password call. Thus, it is clear that password calls were more salient to cowbirds. Moreover, adults also showed activity in brain region associated with song preference. This would suggest password call has additional meanings as cowbirds grow older. In the second experiment, juvenile cowbirds experienced different songs for longer, before their response to one final song was measured. It was found that experience affected the brain activity in juveniles with experience predicting their song preference instead of the singer identity. So, if they had heard blackbird song for a long period, they now preferred this incorrect song. In addition to this, no difference was observed regarding song familiarity anymore.
These findings indicate that cowbird brain has evolved to treat the calls from their own species like passwords to solve the recognition problem. The differences between adults and juveniles’ brain activity can be explained by the different phases of song learning. Initially cowbirds simply need to know what individuals sound like and song learning starts later. This learning is also shown to be affected by experience rather than just having an innate ability. Thus, it makes sense that parasitic chicks leave their nests earlier so as to prevent learning host songs. As cowbirds descended from nonparasitic ancestors, these mechanisms may be undiscovered in other species. These password calls could be the cues that initiate social and learning behaviour in many birds.