Swans.

Hi Gary
From the bond I have formed with a mating pair over the last 6 years I think nibbling is a way they show affection/attachment. Because of how close they get to me now they now nibble my fingers/hands more than anything else and because both of them have bitten me with force in the past, I know the difference between their power levels and when they nibble on my finger they are incredibly gentle. They know it isn't food, they just want to touch.
One of their cygnets from their last set developed a really bizarre habit with me. Instead of nibbling/biting all over like it's parents and siblings it just grabbed the thenar webspace (the skin between thumb and forefinger) on my right hand and then it would simultaneously lower it's body and move as close to me as it could. It wasn't trying to bite me, it was holding and getting as close as it could, like it was literally trying to form some attachment.
It did that 3 times over the time it was there and I had to stroke it to get it to release.