Britannica Sources used in this answer

How do feathers grow and develop in living birds?

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A zebra finch (Taeniopygia guttata) with a bright orange beak, gray head, orange cheek patch, and black-and-white striped tail perches on a branch. — Bird A brightly colored zebra finch (*Taeniopygia guttata*) perched on a branch.

Feathers are unique structures composed of keratin, a fibrous protein, and they develop from specialized follicles located in the skin of birds.1

The Development Process

The growth of a feather begins with a local thickening of the epidermal layer, beneath which a group of mesenchyme cells accumulates. This epidermal thickening bulges outward to form a hollow, fingerlike protrusion with a core of connective tissue. As the structure grows, the shaft branches to form the characteristic barbs and barbules. By the time the feather is fully formed, its primary components—the shaft, barbs, and barbules—consist of keratinized, dead cells.2

Growth Patterns and Maintenance

Feathers typically grow in specific tracts known as pterylae, which are separated by bare areas called apteria. Because feathers are composed of dead tissue, they cannot be repaired and must be periodically replaced through a process called molting. During this cycle, old feathers are shed and new ones grow from the follicles to maintain the bird's plumage for flight, insulation, and display.13