For a long time now we have known that there is a nest behind the basketball hoop in out back yard. But it was not until we saw this sight did we have reason to believe there was two. Last week, we saw two sparrows fighting eachother on the driveway. After a while, they would both fly up to the basketball hoop where they would soon glide down while fighting in the air. Then two female sparrows went and perched up on the garage to watch, and we guessed that the two couples were fighting to see who got to keep the nest behind the hoop. This estimate was also supported by all the twigs and leaf scraps on the ground. One of the birds had the other in a headlock, then they turned over a few times, and the other got the headlock advantage, after which he broke free and they all flew away, including the females. 
    Here is a picture to help you get the idea:

I think they are currently in a mutual headlock…