1) First the male lands and plants his feet on the female's back.
2) Then, he grabs a mouthful of her head feathers for balance!
3) Then, he puts his wings up and performs an impossible twist of his tail to line up cloacas.
This whole process lasts about 1-2 seconds, according to my camera's time stamp. The pair was observed mating several times over the course of a few days. It seemed to happen in the morning and evening more (too hot mid-day?), in short "frenzies" with the male going in for several tries each time. This timing coincided with their recent take-over of a birdhouse after some house sparrows vacated for unknown reasons. I could be wrong, but I think mating only occurs during egg-laying, as there has to be a relatively recent mating to produce a fertilized egg each day. I supposed that's why she always indulged the male, though she didn't always seem into it...
"That was awkward."
As an aside, this is the first time I've realized that male and female tree swallows are quite different looking, the female being much less shiny.