When a bird preens the feathers of another bird, or when two birds preen each other, as is very common in parrots, it is called allopreening. This behavior tends to strengthen the bond between mated pairs but allopreening occurs between unmated birds and even between birds and humans. These two Sulphur-crested Cockatoos, allopreening in Sydney’s Royal Botanical Gardens a few days ago spent quite a long time preening each other’s feathers. I have some experience with allopreening between a bird and a human. My Red-lored Amazon, Bobo, puts her head on my lap so I can preen her head feathers. She turns her head to let me know where she wants to be preened. I do NOT ask her to return the favor!