Courtship is a serious business for these Mallards. Male A has got the female to accept his company (and maybe more), but the other male would like to make her nearer acquaintance. Male A isn’t having any. He pursues the birddogging bird and goes after him with beak wide open. The other flees for the moment, and then hangs in the background. Male A shepherds the female past, always keeping himself between her and the rival.
This scenario probably reflects an overpopulation of males relative to females. Mallards are monogamous for the season, so once all the females are paired up, the unpaired males will have to wait until next year if they play nice. However, Mallards don’t play nice. Bachelor Mallards, alone or in gangs, are known to force themselves on any unguarded female. Male A probably knows this, so he’s sticking close to her, taking no chances.
Note the slight plumage variation between the two males. One of them has a white ring around the neck and a dark breast; the other doesn’t. Mallards interbreed readily with other duck species, creating all kinds of pattern variations to confuse birdwatchers.