Definitely a bird. It's got wings.
I was talking to a builder about getting the outside of my house painted in a couple of weeks' time, when something small and fast flew out from under the apex of the eaves. I've kept watch and seen it fly back in and back out again. Clearly, redecoration work is on hold for the time being. What is the bird likely to be please? I assumed swallow but I have the impression of something small and brown with some white on it. House martin? It flies very very fast. Sorry to be unsure of a description but it really doesn't hang around. The wall in question faces due west.
So, next question: how long before the birds have finished using the nest? I'm assuming painting the wall right under it won't be an option, particularly as I have to get scaffolding put up.
small, with brown and white, could be house sparrow... glimpsing a martin I would have said would appear black. "Very fast" is open to interpretation, but against that, the sparrows would be a permanent fixture (at least, ours are)