Red-bellied woodpeckers (and apparently some other North American woodpeckers) are known predators of songbird eggs and chicks.

I once watched a red-bellied woodpecker behave the same way you have described, Joanne.  It kept lunging deep into a nest hole while a frantic adult downy woodpecker attacked it .  There was lots of noise and commotion.  Eventually, the red-bellied woodpecker plucked out a chick and flew off.   After that, all was silent.

John Dennehy
Baltimore