Bird Scales
The scales of birds are composed of keratin, like beaks, claws, and spurs. They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales and scutes of birds were originally thought to be homologous to those of reptiles and mammals; however, more recent research suggests that scales in birds re-evolved after the evolution of feathers.
Bird embryos begin development with smooth skin. On the feet, the corneum, or outermost layer, of this skin may keratinize, thicken and form scales. These scales can be organized into;
Cancella – minute scales which are really just a thickening and hardening of the skin, crisscrossed with shallow grooves.
Scutella – scales that are not quite as large as scutes, such as those found on the caudal, or hind part, of the chicken metatarsus.
Scutes – the largest scales, usually on the anterior surface of the metatarsus and dorsal surface of the toes.
The rows of scutes on the anterior of the metatarsus can be called an "acrometatarsium" or "acrotarsium".
Reticula are located on the lateral and medial surfaces (sides) of the foot and were originally thought to be separate scales. However, histological and evolutionary developmental work in this area revealed that these structures lack beta-keratin (a hallmark of reptilian scales) and are entirely composed of alpha-keratin. This, along with their unique structure, has led to the suggestion that these are actually feather buds that were arrested early in development.
Reference:
wikipedia.org/wiki/Bird_anatomy
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