Research on the ways in which different species of birds learn to sing is used to illustrate the necessity of taking innate factors into account in studies of behavioral development. Experiments on two species of songbirds are described that reveal innate species differences in responsiveness to tape-recorded songs. Conspecific songs are favored over those of other species. These patterns of innately varying responsiveness provide a basis for the development, not of stereotyped behavior, but of variable, individually learned behavior. The viewpoint is presented that mechanisms that differ innately from species to species, some with general functions, others specialized for particular ontogenetic assignments, provide the necessary substrates with which experience interacts.