Beak
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The beak or bill is an external rostrum structure found mostly in birds. A beak is used for pecking, grasping, and holding, preening, courtship, and feeding young. The terms beak and rostrum are also
The bony core of the beak is a lightweight framework, as seen on this barn owl's skull.
A gull's upper mandible can flex upwards because it is supported by small bones which can move slightly backwards and forwards.
Mallard with an injured beak after a dog attack. Parts of the rhamphotheca have detached, exposing the periosteum.
The sawtooth serrations on a common merganser's bill help it to hold tight to its prey, a fish.
Preening
Videos
Preening is a maintenance behaviour found in birds that involves the use of the beak to position feathers, interlock feather barbules that have become separated, clean plumage, and keep ectoparasites
When preening, a bird (such as this red lory) draws individual feathers through its beak, realigning and re-interlocking the barbules.
Greater flamingo preening in Camargue, France
A humboldt penguin preening at Copenhagen Zoo, Denmark
Birds, like this two-barred crossbill, typically use their beak to gather preen gland oil from the uropygial gland.