We have two female ducks, an Indian Runner and a Khaki Campbell, that are just under a year old and are good layers. Can anyone explain why they sometimes take turns standing on one another's back and pull at the feathers on the back of their head? The one on the bottom never resists. I've seen this described in males related to mating or dominance aggression, but these two ladies seem to get along in every other way. (Even before our other two female Indian Runners were killed by a dog in December, the four ducks exchanged this behavior before laying began.)