This morning we saw 2 Hooded Mergansers flying right at our house. They banked and made a turn and headed towards our Screech Owl/Duck nest box, and we lost sight of them. We did not see if Mrs. Hoodie went into the box. They have doing behavior like this for the last week, flying in and circling around some of our 3 Duck nest boxes. What they are doing is looking for a nest box. When the female selects one, they will still fly in as a pair, then she goes into the box and lays an egg, and he returns to the pond and waits.
He stays glued to her side during this time because he wants to protect his investment. Ducks (and other bird species as well) are notorious for "extra-pair copulations" where they will additionally mate with another bird. So the male does not want another male coming in and mating with the female Hooded Merganser during this time of egg-laying. Then some of the offspring would not be his. So he sticks close to her side and guards her. Of couse, he might not hesitate to mate with another female Hooded Merganser if he got the chance! And she might mate with another male if she got the chance, and even go and lay one of her eggs in another female's nest!
Scientists think this type of behavior, mating with more than one bird, is actually adaptive, because it might mean, literally, a duck would not have "all its eggs in one basket". If a duck mated with additional other ducks, then some of its offspring would be likely to survive if something destroyed its own nest.
What is happening in nature is always more interesting than what we think is happening.