BIRDS sing in dialects as distinct to the avian ear as the difference between a Boston and Mississippi accent is to humans. But there has been a long debate over the significance, if any, of local differences in the songs of a particular species.

According to one major theory, birds are inclined to choose a mate with the same dialect because that helps keep the local group together, thus selectively preserving inborn behavior patterns and capabilities that are most succesful in coping with the local environment.

Scientists studying brown-headed cowbirds have developed a new theory. They believe the female bird cocks an educated ear to the accent of the male in choosing her mate, not so much from chauvinism as from the assurance that the right song means that the male is biologically fit.

The new idea is proposed in the ornithological journal Condor by Stephen I. Rothstein of the University of California at Santa Barbara and Robert C. Fleischer of the University of Hawaii. Their theory is based on observations of the brown-headed cowbird in the eastern Sierra Nevadas. Cowbirds are notorious among bird fanciers because they lay their eggs in the nests of other species, thus shirking the duties of parenthood.