The beak is derived from a cornified specialization of the skin and has the same components. The skin (epidermis, dermis and subcutaneous layer) of birds is generally thinner and more delicate than that of mammals — an important consideration in wound care. It has few attachments to underlying muscles but extensive attachments to bone, particularly the manus (hand) and the pes (foot). This is also true in the case of the beak.
The epidermis is composed of a stratum corneum of dead cells with three “living layers” (basal, intermediate, and transitional). The feather follicles develop in the dermis where blood vessels are found. The subcutaneous layer is formed from loose connective tissue with a variable amount of fat. Subcutaneous fat is especially abundant in aquatic birds for buoyancy and insulation and in passerines during migration as an energy source.
The two main horny structures in birds are the bill and the claws. The bill is also called the beak and technically is called the rhamphotheca. It consists of a horny keratinized (stratum corneum) layer that covers the rostral portions of the upper or maxillary and lower or mandibular jaws. This is the front part of the boney skull that is modified to from this bill. The bill has specializations and the outer stratum corneum is one of those areas that are different from skin.
The stratum corneum is thickened and it contains free calcium phosphate, regularly arranged crystals of hydroxyapatite and keratin-bound phospholipids and calcium. This produces the outer hardness of the bill. The dermis with its blood vessels is directly attached to the underlying periosteum of the jawbones. That is important when trimming beaks to understand this anatomic arrangement.
It is also important with malformed beaks. One of the common problems is that when the young bird or chick is being hand-fed, the pressures exerted by human hands are too strong or not the correct pulling forces so that, as the chick matures, the beak does not grow correctly. That results in it being malformed. That is an argument that Mom knows best the correct forces that need to be exerted on a soft growing beak before is hardens! In macaw chicks, the stronger forces result in a wry beak where the tip of the upper bill grows off to the side.
In young cockatoos, the mom pulls on the upper beak as she locks her beak to the youngster to feed with that pumping action. That lack of proper pulling in that feeding process results in the upper bill shortening in relation to the lower bill.
While the entire bill of most birds is thick and hard, in the wading birds it is more leathery. This long bill is used to probe around in the sand for food items. In these shorebirds, it is only the tip of the bill that is hardened. This hardened tip is called the nail or bill tip organ and is a very specialized structure. It consists of a dermis that is thrown into folds. These folds form a core that contains near the surface of the cornified skin special mechanical receptors with their nerve endings.
These mechanoreceptors are in a perfect position to receive tactile information from the surface during feeding. The principal type of mechanoreceptor of the bill including the bill tip organ of ducks and geese is the Herbst corpuscle. These corpuscles are highly sensitive to vibration and act together for tactile exploration. They are the most common type of mechanoreceptor in birds. These receptors are capable of discriminating food particles from other material.
You can visualize them when trimming the beak of your parrot. As you scrape away the edges of the upper bill you can see on the margins white dots. These are Herbst corpuscles of your bird’s beak. They too have the ability to understand their environment by tactile stimulation — just like when babies put objects in their mouths to understand that particular object better. So does your bird and is part of the normal behavioral repertoire of understanding. That is why some birds like the texture of certain objects and not others. Cockatoos tend to like plastics and hemp or sisal while macaws like hard woods for example. It is good to figure out what you bird likes in terms of tactile structures so that you can incorporate those textures into their foraging.
The beak is constantly growing. It needs to do that so that in the wild the beak keeps up with the wearing away of its structure as the bird is feeding and chewing to get to its foodstuffs. Most of the foods that our birds get in the wild involve lots of chewing and beak manipulation to get to the foods. Often the food has a hard outer shell like our nuts so that there is a lot of grinding that goes on. But in our home environment, the foods are not so hard. The beak continues to grow and flake without those hard grinding foods, so that there will be excess horn. This is the part that is groomed away to not allow build up of these cornified tissues.
Excess tissues may result in infections of the beak. For that reason it is important to have your bird’s beak looked at once yearly by your avian vet. Some birds like African greys do not get this excess scaling like other species. But one thing that is a concern is when you bird has a beak that suddenly takes off growing and grows rapidly. We know that is a sign of liver disease and it is important to get your bird in to your avian vet as soon as possible.
Bird beaks are actively growing structures that bring an important dimension of the external world to your bird so it is important to keep it healthy to make your bird happy! Have fun with your busy beak!