These mallard drakes are very jealously guarding their hens. There is an overabundance of males in the mallard world, and females are in high demand. The same amount of males and females are hatched, but females have a much higher mortality rate, often falling prey to predators when they are trying to guard their nests. So there ends up being quite a shortage of females with fierce competition for them in the duck world.
It’s uncommon to find a hen unaccompanied by a male, and if you do find one, she’s usually running as fast as she can from a whole pile of males. They won’t stop until they catch her. Hens always choose the strongest and best mates, though, those with the “best genes,” territory, and hopefully good looks. That leaves a lot of very unhappy drakes out in the cold.
Aggressive non-paired drake behavior is worse in city parks and crowded areas. Here in Maine, you won’t see as much of it, but you will see it. Not to be outdone or outsmarted, the unpaired drakes will interbreed with other ducks (also aggressively), producing hybrid birds, some of which are fertile. These are definitely opportunistic ducks!
But I love to watch them. In winter, they often sleep right on the ice on the edge of the water. Their little down feathers, hidden deeply inside of the larger feathers, keep them warm and cozy so that they don’t even notice the subzero temperatures. Breeding will start in early spring, and by June the males will leave to go through their molting period. Come October or November, though, they’ll be back looking for their sweethearts again.