Good morning, George~
I may have told this story before, but I first noticed those tertials when I was a young lad. It was a day when big sou'easter blew up and kept us out of our usual cove. So, my Dad just took a long drive - with no particular plans to hunt that day. As we drove over the Fire Island Inlet, I looked down from the bridge and saw a bunch of about 40 big ducks rafted tight together on the water....very dark, heads hunkered down, and these obvious pale markings on the backs aft. I had to ask my Dad - What are those?
As with my later years flying over birds on the water, I realized how different a view other ducks get - and how they ID each other from that perspective.
All the best,
SJS