Preening is a vital activity that cares for a bird's feathers. Feathers are necessary to insulate, protect, and decorate a bird. If not properly cared for feathers become dirty and pest-ridden. Feathers in good condition are necessary for this female Common Merganser to fly efficiently and swim without becoming water-logged.
A gland near the tail provides an oil that she rubs on her feathers. She rubs her bill in the oil and then distribute it to other areas while preening. Evidently, the oil is not absolutely essential for waterproofing--the feather structure itself does that. But the oil does help keep the plumage healthy.
It's hard to reach everywhere!
She grasps feathers in her bill and removes dirt and insect pests. At the same time she pulls the feather through her bill and re-forms the feather vanes--zipping up feathers whose webs may have split apart.
All finished... beautiful!