Dustin Reichard and colleagues show that some female Dark-eyed Juncos sing when riled up about female intruders!
Popular coverage by New Scientist here: https://www.newscientist.com/article/2151727-female-birds-that-used-to-be-silent-are-now-singing-like-males/
Go here for the full scientific article: http://onlinelibrary.wiley.com/doi/10.1111/jav.01566/abstract