Front Range Wildlife Photographers Facebook group. It seems odd to me that raptors are more skittish than most of the birds they prey on.
EDIT: Luis A. Ramirez from the group concurs that it's an eagle.
EDIT: Carol Wade from the group says they're hooded mergansers.
EDIT: Luis to the rescue again -- red-tailed hawk.
I surprised myself by spending nearly three hours in the park. Granted, most of that was in my car. But my toes still haven't completely thawed out!