a template representation of the species-specific song. The sensorimotor phase follows and may overlap with the sensory phase. During the sensorimotor phase, young birds initially produce variable, rambling versions of adult song, called subsong. As learning progresses, the subsong is replaced with a more refined version containing elements of adult song, called plastic song. Finally, the song learning crystallizes into adult song. For song learning to occur properly, young birds must be able to hear and refine their vocal productions, and birds deafened before the development of subsong do not learn to produce normal adult song. The sensitive period in which birds must be exposed to song tutoring varies across species, but typically occurs within the first year of life. Birds in which song learning is limited to the initial sensitive period are referred to as closed-ended learners, whereas some birds (e.g. canaries; Serinus canaria), continue to learn new songs later in life, and are called open-ended learners. Some species of birds, such as the brown-headed cowbird (Molothrus ater), parasitize other bird species, laying their eggs in the nests of other birds such that the heterospecific bird raises the chicks. Although most birds acquire song learning within the first year, brown-headed cowbirds have a delayed sensitive period, occurring approximately one year after hatching. This may be an adaptation to prevent the young birds from learning the songs from the foreign bird species. Instead, the young birds have a year in which to find conspecifics, and learn their own species-specific song. Birds are generally predisposed to favour learning of conspecific songs, and will typically preferentially learn the song form conspecific animals rather than heterospecifics. However, song learning is not completely restricted to within-species songs. If exposed to heterospecific birds of another species in absence of same-species birds, young birds will often adopt the song of the species to which it was exposed. Although birds are capable of learning song production purely from audio recordings of birdsong, tutor-student interaction may be important in some species. For example, white-crowned sparrows (Zonotrichia leucophrys) preferentially learn the songs of song sparrows (Melospiza melodia) when exposed to recordings of white-crowned sparrows and live song sparrows. In other words, the interactive nature of a live tutor seems to trump the familiarity of the recordings from conspecifics. Cultural transmission of whale song While vertical transmission (parent-offspring) is a common element of song learning, horizontal transmission among animals of