Author response image 4. Learning trajectories demonstrate that birds’ songs change over song development.

Learning trajectory is calculated by quantifying acoustic similarity between birds’ songs over the course of the experiment and their song on the day of sacrifice (~65d). Upward trajectory indicates the songs for all three virus groups change over time. In agreement with the work of Heston & White, (2015), the FoxP2.FL group’s (black) song changes over time but, as illustrated in main text Figure 3D, not to match the tutor’s song.