I got some good video of the cardinals on their nest, including a segment where the male feeds the female. That is something they do as part of pair-bonding and during nesting. It’s very practical then because it allows her to stay on the nest and incubate full time while he forages and brings food for her. Unlike wood ducks where the males are out of the picture once incubation starts, cardinals work as a team during incubation and later, to feed the young ones.
There are now 3 eggs, so I now think that incubation did not quite start yesterday even though she was on the nest a lot. We probably found the nest just as they were wrapping up the egg-laying part. There might even be one more egg soon, although 3 is a typical number. Incubation time is about 12 days so the eggs probably will hatch around mid-May, about the same time that the wood ducks and mergansers are due! But for the cardinals there will then be a week or two of watching them grow.