If feathers did not evolve first for flight, what was their purpose?

Tail feather of a blue-fronted Amazon parrot
At Institute of Zoology and Zoological Museum, University of Hamburg

In an 1860 letter Charles Darwin despaired over how natural selection could account for such an impediment to flight as a peacock’s train. He later came up with sexual selection: Gaudy peacocks please peahens and pass on their genes.
Composite of three images
At Peabody Museum of Natural History, Yale University

King bird of paradise
Disk tail-feather tip, wobbles during display
Courtesy Peter Mullen, Ph.D.

Gray peacock pheasant
Tail covert, fan display
Institute of Zoology and Zoological Museum, University of Hamburg

Ostrich (chick)
Body feathers, first and second stage, insulation
Institute of Zoology and Zoological Museum, University of Hamburg

Red bird of paradise
Flank plumes, display
Institute of Zoology and Zoological Museum, University of Hamburg

Spotted eagle-owl
Wing feather with serrated edge, muffles sound
Courtesy Peter Mullen, Ph.D.

Scarlet macaw
Wing covert feather, flight
Institute of Zoology and Zoological Museum, University of Hamburg

Golden-headed quetzal
Tail covert, display
Institute of Zoology and Zoological Museum, University of Hamburg

Golden pheasant
Head crest, display
Courtesy Peter Mullen, Ph.D.

Northern flicker
Tail feather, assists in climbing
Institute of Zoology and Zoological Museum, University of Hamburg

Red-crested turaco
Wing feather with copper-containing pigment, flight and display
Courtesy Peter Mullen, Ph.D.

The Eyes Have it
The male great argus of Southeast Asia is a fairly drab pheasant—until he dances before a female with his enormous wing feathers fanned open, revealing the spectacular inner surface shown on this four-inch section. Hundreds of jewel-like ocelli, or eyespots, keep hens enchanted.

At Institute of Zoology and Zoological Museum, University of Hamburg