Odd Duck Behavior

Please help me understand the attached behavior. I took these photos yesterday outside Chicago. I know it is Spring and ducks are mating so it appeared strange to see 2 male ducks mating? These 2 appeared to be mating for an extended time which was precipitated by the traditional head bobbing. This did not appear to be a display of aggression between males fighting over a hen. Can someone who understands waterfowl help me out with an explanation of what was happening here?

Thanks

Rick

I attached another pciture to show it was not a briefly captured moment between 2 rivals.

So sorry, here is the other picture.

"Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population."
Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, St. Martin's Press, 1999; ISBN 0312192398

Arne,

That would explain the bahavior. Thanks for the information.

Rick

The rate is even higher among populations of feral birds where birds are highly concentrated.

Environmental toxins have also been shown to increase the incidence of male-male mating behavior in ducks.