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The male zebra finch produces learned song and long calls while the female does not. This difference in behavior is believed to result from the action of sex steroids on brain areas responsible for vocal production and learning. In this study, the female zebra finch was used to explore further the specific role sex steroids play in vocal masculinization. We show that estradiol (E2) treatment at birth was sufficient to masculinize the vocal behavior of female zebra finches. Thirteen of 18 females treated with E2 as nestlings produced song-like vocalizations. Fifteen of 18 produced long calls with male-typical features. The degree of masculinization varied between individuals. Of the 15 early E2 females that produced at least one type of male-like vocalization, 7 showed evidence of vocal learning from their tutors. The ability of E2 to cause masculinization of vocal behavior was age dependent: treatment from birth was most effective, treatment at 20 days of age was partially effective, and treatment in adulthood was ineffective. The effect of subsequent testosterone exposure in adulthood differed depending on the quality of the vocalization produced after E2 treatment alone. These results suggest that E2 may play a more important role than previously thought in the development of sex differences in vocal behavior. Furthermore, this study demonstrates that exogenous E2 treatment alone can induce vocal learning.

1. Microstimulation (trains of biphasic current pulses at 50-400 Hz lasting 2-4 s) was delivered unilaterally to known vocal control areas in the brains of zebra finches and canaries to elicit vocalizations. 2. Simple vocalizations were elicited from the midbrain, and the lowest thresholds were obtained from the dorsomedial nucleus of the intercollicular complex (DM). 3. Vocalizations elicited from forebrain vocal control nuclei higher vocal center (HVC) and robustus archistriatalis (RA) were complex, with features specific not only to the species, but to the individual bird's own learned song. 4. Complex acoustic features depended on innervation of the bird's vocal organ and were lost when the tracheosyringeal nerve was cut. 5. We suggest that stimulation of the forebrain vocal pathway activates a dedicated neural circuit that generates the temporal structure of song and whose specific pattern of activity is programmed during sensorimotor learning in each individual.

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