We have a male mallard duck that has decided to perch himself atop of our house roof. Although he has not done any damage we feel there is a reason he is there. We have searched are yard for a female/or another duck but have found nothing. We chase him away but he keeps coming back. Would there be a reason he is doing this? Maybe he is an angle in costume.

Your roof probably provides him with an excellent view of the surrounding area and keeps him safely out of reach of most predators. There may or may not be a hen on a nest nearby. Male mallards do not help out with the incubation of eggs, so his presence wouldn't indicate that there is a nest. As long as he is not causing problems, I'd advise to just enjoy his company while he decides to stick around.