It is definitely the Redstart that is flying around and perching under the roof of the open barn which is the Martins' nesting area. I have spent quite some time watching it over several days. It flies onto the roof, as in the photos, then under the roof, perches on beams from where it often flies vertically to the floor and then up again, flies in and out of the crevices occupied by Martin nests, all the time making its clicking call. If the Martins are around it flies with them. I'm not confusing it with more than one bird.