Nestling white-crowned sparrows (Zonotrichia leucophrys nuttalli) were hand-reared in sound-isolation chambers under a variety of conditions. The songs of total isolates were compared with songs of birds tutored with song, and the number of inputs sufficient for a bird to produce a normal song was explored. The flexibility of the song learning system was investigated with a range of tape-recorded tutor songs: alien dialects, alien subspecies, alien species, alternating alien dialects, and an aberrant song. Adult songs were obtained for 40 males and 7 testosterone-injected females. All of the tutor songs could be learned. Also, some birds learned elements of an alien species' song. Birds tutored with two songs copied one or the other, were bilingual, or sang a hybrid of the two. No bird presented with fewer than 120 songs learned the tutor song; 2 birds tutored with 252 songs copied the tutor song. It is concluded that the song learning system is quite flexible, hat the results obtained with tape-tutors are very different from those with social tutors, and that there may be an interaction between total number of song inputs and the number presented on a single day. Some implications of these data for physiological mechanisms and the possible functional significance of the acquisition system are discussed.