The PeeWee is a very terratorial bird. It's seeing its own reflection in the window, and it's too dumb to realise that it is itself that it is seeing. Mind you, nice work CC on having lovely clean windows
After a while he'll give up (may take a few days to weeks), but you can always place a mirror out in the yard, near the window. He'll see that and hassle the mirror and leave your window alone.