Avian folly, but cute

Pied wagtails, sometimes two at a time, have been flying up to my window, landing carefully on the sill, and tapping on it with their beaks all morning.

I could think of this as attacking the wagtail in the mirror, but in that case I don't see why the wagtail appears indifferent to the real wagtail next to it while it taps away.

PS: they're long-tailed tits not pied wagtails; fluffier, a bit smaller, brown bits on the side of the neck - I had wondered whether juvenile pied wagtails had the brown bits, but they don't.