My friend heard a song by Duffy and when she sang
what he heard was
"Mercy" makes better sense, people don't often beg for birdseed, but that's not necessarily how Mondegreens work. In The Language Instinct Steve Pinker mentions Mondegreens and notes that the mishearing is often fits the context less well than the original word and this seems be an example of that.
Yet perhaps a singer begging for birdseed does have some kind of underlying logic, when you think about it.