We investigated real time changes in brain activity during active vocal learning in the zebra finch songbird. The song nucleus HVC is required for the production of learned song (Nottebohm et al., 1976; Simpson and Vicario, 1990). To quantify the relationship of HVC activity and behavior, HVC population activity during repeated vocal sequences (motifs) was recorded and temporally aligned relative to the motif, millisecond by millisecond. Somewhat surprisingly, HVC activity did not reliably predict any vocal feature except amplitude and, to a lesser extent, entropy and pitch goodness (sound periodicity). Variance in 'premotor' HVC activity did not reliably predict variance in behavior. In contrast, HVC activity inversely predicted the variance of amplitude, entropy, frequency, pitch, and frequency modulation. We reasoned that, if HVC were involved in song learning, the relationship of HVC activity to learned features would be developmentally regulated. To test this hypothesis, we compared the HVC-song feature relationships in juveniles in the sensorimotor 'babbling' period and adults. We found that the relationship of HVC activity to variance in frequency modulation was developmentally regulated, with the greatest difference at an HVC-vocalization lag of 50 ms. Collectively, these data show that, millisecond by millisecond, bursts in HVC activity predict song stability on-line during singing, whereas decrements in HVC activity predict plasticity. These relationships between neural activity and plasticity may play a role in vocal learning in songbirds by enabling the selective stabilization of parts of the song that match a learned tutor model.