Chemistry of Birds

The Chemistry of Birds (13): Keratin

September 28, 2025 by Kai Pflug No Comments

Keratin is a family of structural, fibrous proteins. These proteins are the primary components of hair, nails, feathers, claws, hooves, and the outer layer of skin in animals.

There are two main types: alpha-keratin and beta-keratin. Both are found in birds, but the beta-keratin is specific to birds (and reptiles) and is responsible for many of the unique properties of birds.

Specifically, many parts of a bird’s body (feathers, beaks, and claws) are composed of both alpha-keratin and beta-keratin, and the individual mixture of these two keratin types is partly responsible for the huge variety in these body parts.

Alpha-keratins are less common in birds than in mammals. Chemically, these are proteins (large molecules made up of amino acids joined in long chains) that form coiled, flexible structures.

In contrast, beta-keratins are smaller proteins that form rigid, stacked sheets. This makes body parts containing a large proportion of beta-keratins particularly tough and durable.

One subfamily of beta-keratins is the feather-beta-keratin subfamily. Chemically, these keratins have some distinct characteristics resulting in distinct material properties:

  • They are comparatively small proteins of only about 100 amino acids
  • They contain a core region of about 34 amino acids that is highly consistent across different beta-keratins. This core is rich in two amino acids, glycine and tyrosine, which are a major reason for the rigid structure.

Glycine

Tyrosine

  • As mentioned above, the polypeptide chains of beta-keratins are arranged into beta-pleated sheets. These sheets are formed by hydrogen bonds between the carbonyl oxygen of one peptide bond and the amide hydrogen of another, creating a highly stable, folded structure. The sheets then stack together to form strong filaments.
  • The stacked beta-pleated sheets are further strengthened by disulfide bridges formed between cysteine units (cysteine is another amino acid). These covalent bonds link adjacent protein chains, creating a highly cross-linked, rigid, and insoluble material. The high density of these disulfide bonds is a key factor in the higher toughness and chemical resistance of avian beta-keratin compared to mammalian alpha-keratin.

Cysteine

  • The chemical nature of beta-keratin also contributes to the structural coloration of many bird feathers. The protein self-assembles into intricate nanostructures within feather cells. These structures, along with air cavities, coherently scatter light, producing vibrant, non-iridescent colours like blues and greens. This process is distinct from pigment-based coloration and is directly dependent on the precise arrangement of the keratin and air. It thus requires a highly regular structure that is provided by the beta-keratin.

Interestingly, while feathered non-avian dinosaurs likely produced avian beta-keratin, they lacked specialized feather beta-keratin. This keratin has a higher elasticity and thus is more suited for flight feathers. Thus, this specialized beta-keratin may have played a role in the development of flight.

Illustrations:

Written by Kai Pflug
Kai has lived in Shanghai for 21 years. He only started birding after moving to China, so he is far more familiar with Chinese birds than the ones back in his native Germany. As a birder, he considers himself strictly average and tries to make up for it with photography, which he shares on a separate website. Alas, most of the photos are pretty average as well.He hopes that few clients of his consulting firm—focused on China’s chemical industry—ever find this blog, as it might raise questions about his professional priorities. Much of his time is spent either editing posts for 10,000 Birds or cleaning the litter boxes of his numerous indoor cats. He occasionally considers writing a piece comparing the two activities.