Thanks for such a quick response. While I don't know if the young male (Spencer) is an offspring of Bella and her mate, I do know that Spencer is almost 2, as he arrived at my dock on a 550 acre lake last year around this time still sporting a great deal of his baby brown feathers. Two weeks later, two other birds his age arrived; for a few months they were a threesome, and then he became the odd man out. I know that Bella is at least 5, maybe older, and I don't know anything about her mate.
This leads me to another question: Can mute siblings become mates, and if so, does that pose a problem with their offspring?
As always, thank you so much for the information!
Pat