Feathers give meaning to the word bird. ~ Gary Kaiser
Little of the visible part of a bird is living tissue. Feathers are made of keratin, a tough fibrous protein. For protection during flight, avian eyes are covered in a sheet of keratin.
Feathers evolved from reptilian scales 100 million years before they served any locomotive purpose. Birds still have scales on the lower part of their legs and feet.
Like reptiles (and, later, birds), dinosaurs had highly differentiated color vision (tetrachromacy): much superior to that of humans and other mammals. This led to feathers summing up to something more: plumage, a colorful coating. Insulation and coloration were feathers first purposes.
The membranous wings of bats are effective, whereas the feathered wings of birds are splendidly sophisticated. No other animal has so much non-living tissue that does so much.
The similarity of feathers among both modern birds and their distant ancestors suggests that evolutionary constraints on feathers have always been strong. ~ Gary Kaiser
Flight wears on feathers. Every bird goes through a series of plumage changes during its life.
Besides flight, feathers protect, insulate, and communicate. No other body structure serves such diverse purposes. Plumage often displays a readiness for reproduction, and, in males, acts as advertisement for the act.
Darwin and his followers long assumed that the 1st function of colorful feathers was a dancing display of color for mating: that sexual selection made males put on more sexy plumage. But male birds with multiple mates tend to be drabber than their female counterparts. Male red-winged blackbirds might have up to a dozen mates, but they are less colorful than their consorts.
Avian females have varied between drab and colorful through time. To be less conspicuous to predators, migratory birds tend to be less vivid.
Plumage is important for social signaling and camouflage, not just sexual selection. English naturalist Alfred Russel Wallace, a contemporary of Darwin, thought as much. The pulchritude of avian plumages owes to a confluence of factors.
Ecology and behavior are driving the color of both sexes, and it is not due to sexual selection. Both natural and sexual selection have been influential, but they have generally acted on 2 different axes: sexual selection on an axis of sexual differences and natural selection on both sexes for the type of color (for example, bright or dull). ~ Canadian ethologist Peter Dunn et al
For all the variety of roles, modern avian plumage comes in 4 basic types. Each differs by lifestyle.
Flightless birds make do with basic body coverings of very few feathers with specialized shapes. Nonetheless, these feathers range from the simple hair-like structures on kiwi to fluffy ostrich plumes.
In contrast, flying birds have many specialized feathers, albeit in 2 basic types. Terrestrial birds of all sorts have soft and flexible plumage; typically, a small number of relatively large feathers. Ducks and seabirds are at the other end: firm, densely packed plumage from a copious number of small feathers. Penguins take this to an extreme, with feathers so tightly packed that they create a waterproof surface, much like the scaly covering of ancient reptiles.
Owls can fly almost completely silently. Their wings are broad and curved – ideal for slow gliding – and abundantly veined with velvety down plumage that absorbs sound. The feathers at the edge are serrated, effectively breaking up and smoothing out air turbulence, like a comb slowly untangles knots.
The compound responsible for feathers – keratin – comes in chemical varieties. Families of birds independently developed specific keratins best suited to the purposes to which they are put.