Window shopping?

Backblipping ... 

We've been staying for a week in a cottage near Gairloch. This long-tailed tit welcomed us by flying up to the window repeatedly and tapping on the glass. It did this every evening for half an hour or so.

I have no idea why - any ideas? It didn't seem to be catching insects, just tapping its beak on the glass. It didn't go for the same part of the frame, either, which made it hard to capture. (This is the best I could manage from several attempts over the week.) It did have a preferred window, although it went to the others once or twice.