(A) Seven different PV oligos (symbols) in the top 100 candidate gene transcripts were differentially expressed between vocal learners and non-learners in the arcopallium. (B) Example PV oligo that showed lower PV expression in nXllts compared to SSp in hummingbird, dove, and quail. Y-axes, log2 fold expression ratio of the arcopallium song nucleus or ciA versus the adjacent iA, or SSp relative to nXIIts; ratio = 0 means no difference between brain areas. Clone IDs of cDNAs that the oligos recognize are in http://songbirdtranscriptome.net/and http://aviangenomes.org. ZF, zebra finch; BG, budgerigar; H, hummingbird; D, dove; Q, quail. **p<0.01 paired t-test on each oligo of each group of species that share a trait (n = 9 vocal learners; n = 5 vocal non-learners; empirical Bayes adjusted paired t-test; the data shown is for averages of each species). **p<0.01, unpaired t-test, combined values of PV oligo variants.