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.