Summary of the results of this study and proposed theory.

<p>A. Schematic drawing of the known vocal pathway in songbirds (a) and the putative adjacent non-vocal motor pathway in all birds (b). Movement-associated areas adjacent to the posterior vocal nuclei (HVC, RA, NIf, and Av) in (b) are drawn with dashed lines to indicate that they are lateral to the plane of the section shown. Lines and arrows in (b) are inferred from our compilation of the literature on tracers placed adjacent to the vocal nuclei (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001768#pone-0001768-t003" target="_blank">Table 3</a>). White arrows: connectivity of anterior vocal pathway (a) and proposed adjacent anterior motor pathway (b). Black arrows: connectivity of posterior vocal pathway (a) and proposed adjacent posterior motor pathway (b). Not all known connections are shown; in particular, the anterior mesopallium connections have not been determined in songbirds, a DLN to ASt connection appears to be weak in zebra finches, and RA and LAI also projects to other sub-telencephalic areas (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001768#pone-0001768-t003" target="_blank">Table 3</a> and references therein), and connectivity of PLN and PLMV with other movement-associated regions is not known. Different background colors designate different cerebral brain subdivisions. B. Diagram comparing brain organization in the three vocal learning groups and in a vocal non-learner as a proposed common ancestor. We hypothesize that by independent evolution, the vocal nuclei (light colored boxes) of recent vocal learners originated from the movement-associated brain areas (dark colored boxes) of the common ancestor. Relative sizes and positions of brain areas are approximate. The parrot posterior regions (LAI, SLN, LAN, and LAMV) are more anterior and laterally than the corresponding areas in the other species. Hummingbird PLN and PLMV are highlighted with dashed lines to indicate that they were only examined in a few birds. Songbird HVC is drawn as adjacent to both DLN and PLN, and thus, it is ambiguous as to which region it could have evolved from. Color-coding in panel (B) reflects the coding of panel (A).</p>