ScienceDaily reported on a study that birds that have the most variation in weather may also have the most variation in song. Areas with greater precipitation have denser vegetation. As birds arrive to their breeding areas their may be little foliage on trees, but as the season progresses, foliage density increases, which changes how sound travels. Birds that have more variation in their songs are better able to cope with the changing acoustics of their environments.
In 2008 a study on mockingbird song and environment showed a similar relationship. The new study shows the pattern across many species.
Read more at ScienceDaily Birds that live with varying weather sing more versatile songs.