Birdsong, in non-tropical species, is generally more common in spring and summer when males sing to attract mates and/or defend territories. Changes in the volumes of song control nuclei, such as HVC and the robust nucleus of the arcopallium (RA), are observed seasonally. Long photoperiods in spring stimulate the recrudescence of the testes and the release of testosterone. Androgen receptors, and at times estrogen receptors, are present in HVC and RA as are co-factors that facilitate the transcriptional activity of these receptors. Thus testosterone can act directly to induce changes in nucleus volume. However, dissociations have been identified at times among long photoperiods, maximal concentrations of testosterone, large song control nuclei, and high rates of song. One explanation of these dissociations is that song behavior itself can influence neural plasticity in the song system. Testosterone can act via brain-derived neurotrophic factor (BDNF) that is also released in HVC as a result of song activity. Testosterone could enhance song nucleus volume indirectly by acting in the preoptic area, a region regulating sexual behaviors, including song, that connects to the song system through catecholaminergic cells. Seasonal neuroplasticity in the song system involves an interplay among seasonal state, testosterone action, and behavioral activity.

In seasonally breeding songbirds, variations in testosterone and song correlate with volume changes in brain nuclei associated with song, including the HVC. The authors tested whether singing can lead to activity-dependent increases in HVC volume by examining song output in starlings (Sturnus vulgaris). The authors manipulated males' environments so that only some were dominant with nestboxes, whereas others were not. Some of these males thus sang at higher rates and had larger HVC volume than others. The study was conducted over 2 years. In 1 year, males selectively occupied nestboxes but did not sing. HVC volume did not differ in these starlings, indicating that nestbox possession alone cannot increase HVC. The findings suggest that changes in song nuclei volume can be driven by changes in singing.

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