I was sure it wasn't swallows as they won't come that close to the house. I doubt it would be robins so suspected the culprits to be morning doves or sparrows. I suggested a trick the marinas use and that is fishing line. At the marinas they string the line from tall pole to pole. What I don't like about this method is without any background the line blends into the sky causing some birds to get entangled. On the small scale, household use stringing fishing line about 1 - inch above where you don't want the birds to nest is quite effective. They are going very slow trying to get into a nesting site so recognize the barrier the fishing line presents and will move elsewhere. This is an inexpensive, almost unnoticeable way to solve a problem without causing a problem. And that is rule number to for battling any enemy inside or outside, do no cause a greater problem!