Schematic of the song control system of songbirds and seasonal features of the canary song. (A) HVC and RA are essential nuclei of the descending motor pathway of the song system that controls song production. All interconnected areas are parts of loops that feed back to the song motor pathway. The androgen receptors (red dots) are expressed in both the HVC and RA of canaries, while only HVC contains estrogen receptors (green dots) among song areas. Both areas express the 5α-reductase gene (blue squares) but only HVC expresses aromatase (yellow squares). The entopallium (ENT; the bird visual cortex) expresses neither androgen or estrogen receptors nor androgen- or estrogen-producing enzymes and was used as a contrasting brain area [20-23] (this study). (B) Song features such as song length, the number of non-repeated syllables, the number of sexy syllables and the maximal repetition rate of syllables change seasonally while the syllable repertoire of canaries does not change between their breeding and non-breeding season. The abundance of song segments with high repetition rates is a sexually attractive song pattern in canaries. Plotted are the percent differences between the breeding males (control) producing high testosterone levels and the non-breeding males producing low testosterone levels (redrawn after ).