A knock-down fight between two male House Sparrows.
Bill-biting seemed to be the main strategy here. I wonder if this is to immobilize this fearsome weapon? (In social media discussion on another topic recently, an ornithologist noted that these birds can inflict a painful bite.)
In his classic The House Sparrow, J.D. Summers-Smith notes that he observed males fighting for up to an hour over nest sites.
Tomorrow: the prize.
Backyard and Beyond 
They are very aggressive.