(A) Mean amplitude modulation (AM) correlation plot for all animals. The AM distribution demonstrated a gradual consolidation during the 1st testosterone treatment (T1+), and displayed a short phase of re-development during a 2nd testosterone treatment (T2+) (B,C). Both mean frequency (D–F) and wiener entropy (G–I) demonstrated a gradual development during the 1st testosterone treatment (T1+) followed by an immediate recovery during a 2nd treatment (T2+). Dmax could not be calculated for T2+ in F,I, as we observed no developmental increase of these song features during the 2nd testosterone treatment. Grey bars in A, D and G indicate the SEM. Columns in B,C, E,F, and H,I represent the mean ± SEM and open circles indicate individual data points (**p<0.01, ***p≤0.001, paired t-test, n = 6 animal).