Seasonal changes in gonadal hormone levels of adult male canaries and their relation to song.

Samples of song and blood levels of three gonadal hormones, T, DHT, and E2, were taken at monthly intervals from six adult male canaries over a period of 1 year, as these birds went from 12 to 24 months of age. Song variability and addition of new syllable types were maximal during the summer and early fall, with a peak in September. A secondary peak in new syllable acquisition occurred in March. Blood T levels were particularly low during July-August and during February. Thus, a lowering of blood T levels preceded by about 1 month the two marked peaks in new syllable acquisition. Blood levels of the two other hormones were related less obviously to song learning peaks. It is hypothesized that a concurrence of hormonal, neural, and behavioral changes facilitates song learning in adulthood.