Why is this mallard so mad?

In our neighbor's backyard, a pair of mallards, husband and wife, were relaxing in the sun. Mr. M. was resting comfortably with his lady love nearby, until...

...a second male mallard invaded his territory. So mallard #1 rose to his feet and stared menacingly at the other male, but...

...that didn't do the trick. Male #2 didn't retreat, so our hero put on his furious face and dashed toward #2, and chased him away.