Dennis Paulson supplied just the information I was looking for, namely:

Temperate-zone ducks don't maintain year-round pair bonds; they
form them anew each winter on the wintering grounds. They stay
together
in part because they constantly display and frequently copulate.
This is
pair-bond maintenance a la Homo sapiens. The pair then flies to the
female's choice of breeding grounds, copulation now results in
fertilization, she lays her eggs, he deserts her, and that's the
last time
they see each other until perhaps they might meet the next winter.

The point of my previous message was that since I observed displaying in the
depths of the winter months, it didn't appear to be a sign of spring, a
behavior brought on by more daylight and more hormones flowing. If I
understand you correctly, Dennis, displaying, pairing, and copulating in
these ducks isn't associated with springtime, and it's only in the spring
when the gonads have enlarged that copulation results in fertilization.

Burt Guttman
The Evergreen State College 360-867-6755
Olympia, WA 98505 guttmanb at evergreen.edu

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