Birds' mating ritual?

I'm no expert on birds but I'm guessing someone on here might be...

A couple of times this week I've watched a pair of sparrows outside my back door, and wondered what was going on. One of them, I'm guessing the female, puts her tail in the air and quivers her tail feathers. The other then hops up and pecks her on the bottom. I'm guessing this is some sort of mating ritual - but what IS he doing? Surely sparrows aren't into oral sex... or are they?!