Sex in the Backyard

The avian kind, that is! Yesterday morning, I noticed that a Mourning Dove was sitting on a nest that was placed atop a fence near the garage. Later in the day, I was sitting in the backyard watching the dove on the nest--not much happening there! Suddenly, I noticed two doves perched together on a horizonal branch of a red maple. They were engaged in courtship behavior (billing, etc.). Then, within full sight of the bird sitting on the nest and not more than 30 feet away, one of the doves in the tree mounted the other one and they copulated briefly. Was one of these birds the (promiscuous) mate of the bird sitting on the nest? Or did these two birds represent a second pair establishing a pair bond and breeding territory in close proximity to an already established territory?