A few years ago, an avid Pennsylvania birder discovered an individual that looked like a Rose-breasted Grosbeak but sang like a Scarlet Tanager (listen here). Scientists who analyzed the bird's DNA discovered that it was a one-year old hybrid male with a grosbeak mom and tanager dad.
A further analysis of the bird's song showed that it was highly similar to a Scarlet Tanager. The study's lead author noted “Something people may not understand is that when we analyze birdsongs, we're not actually listening to them. We're looking at them. We're looking at wavelengths of the sound — or the ‘spectrogram’ is a more accurate term — and we’re actually measuring visual components of a soundwave to analyze the song.”
The authors suggest that this individual learned its song from its father or nearby Scarlet Tanagers during its first breeding season.