All cormorants have preen gland secretions that are used ostensibly to keep the feathers waterproof. Some sources[10] state that cormorants have waterproof feathers while others say that they have water-permeable feathers.[11][12] Still others suggest that the outer plumage absorbs water but does not permit it to penetrate the layer of air next to the skin.[13]
https://en.wikipedia.org/wiki/Cormorant
4:58 for feathersBut true waterfowl, such as ducks, geese, swans, lubricate themselves with a special oily ointment secreted by the caudal gland (a sebaceous gland located above the tailbone). In this process of waterproofing, the bird squeezes the oil from the gland with its beak, takes it and gradually lubricates one part of the body after another. The head is the most difficult to handle. At the very end, it has to be rubbed along the already lubricated back or smeared with a foot.
https://www.lvm.lv/jaunumi/3132-dzivnie ... oka-tiribu
Google translation from latvian

4:59 for head
