While watching a female common merganser (Mergus merganser) swimming on Hat Creek below the #1 Hydro Plant (Lassen County CA), I was surprised to see her approach and climb onto a partially submerged snag where two Canada geese (Branta canadensis) were resting. I fully expected a territorial battle.
Much to my surprise, the merganser walked up to the geese, who were not at all disturbed by the intruder. For several minutes the merganser and geese stood next to each other preening themselves, after which the merganser settled down with the geese and all napped or rested together. The merganser, if not tucked directly against the geese, was very close. The three waterfowl remained together until I left after about fifteen minutes.
Canada geese, in my opinion, can be very territorial and aggressive to encroachers during the mating/breeding season. Their acceptance of the lone female merganser was surprising to me.