Darkfield photomicrographs showing <i>PVALB</i> and <i>ZENK</i> mRNA expression in male zebra finch (a songbird) and Anna’s hummingbird brains.

<p>(<b>a</b>) High <i>PVALB</i> expression in HVC and RA song nuclei, and moderate expression in Ai in the male zebra finch; (<b>b</b>) <i>PVALB</i> expression in LMAN song nucleus in the zebra finch; (<b>c</b>) Vocalizing-driven <i>EGR1</i> expression in HVC and RA song nuclei in a male zebra finch that was singing alone in a sound box; (<b>d</b>) Vocalizing-driven <i>EGR1</i> expression in MO, LMAN, and Area X song nuclei of the same zebra finch as in (c); (<b>e</b>) <i>PVALB</i> expression in VLN and VA song nuclei of the male Anna’s hummingbird; (<b>f</b>) High <i>PVALB</i> expression in VAN in the Anna’s hummingbird not vocalizing; (<b>g</b>) Vocalizing-driven <i>EGR1</i> expression in VLN and VA song nuclei of the Anna’s hummingbird that was also flying and feeding, in the early morning (1h after sunrise); (<b>h</b>) Vocalizing-driven <i>EGR1</i> expression in the VAN song nucleus, except within an internal region dorsal posterior region (VANp). Sections are in the coronal plane; medial is to the left, dorsal is top. Panels are sections collected from animals used in Hara et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118496#pone.0118496.ref038" target="_blank">38</a>] (panels a, b, e, f) and Feenders et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118496#pone.0118496.ref035" target="_blank">35</a>] (c, d, g, h). See list for Abbreviations. Scale bars = 2mm.