Male songbirds perform songs to attract mates, meaning the brain nuclei that control song are over 5x bigger in males. Giving newborn females testosterone or estradiol allows these nuclei to develop larger (in line with the organizational hypothesis). If given more testosterone in adulthood, female songbirds can sing just like males. However, canaries only require hormone treatment in adulthood to start singing. The explanation is that birds that reproduce year-round need both rounds of treatment, but if they are seasonal breeders, the nuclei grow in the spring alongside testosterone and singing and all decline in the fall.