Some other selective factor must have operated to give rise to feathers strong and large enough to have become pre-adaptive to flight. Display translates directly into reproductive success via the intraspecific communication used in sexual selection, and it is important in repelling predators. We propose, as a working hypothesis, that display was important in the evolution of feathers from scales. Feathers are scales which have been elongated and raised as large planar structures. A sophisticated dermal musculature allows them to be raised, lowered, and waved.
Birds go to great lengths to maintain plumage in good condition.
We suggest that the enlargement and the planar structure, the motility and the maintenance of plumage all arose as part of a display function during the first appearance of proto-feathers. If feathers arose for display, they should be most strongly developed on areas that are prominent and easily displayed. They should be movable for maximum effect, to increase apparent size and to draw attention to agility. If display arose in a small bipedal archosaur, it would reasonably have been best developed on the arms and tail, appendages that are comparatively high on the organism, and less intimately associated with locomotion and feeding than the feet and head. Evidence from Archaeopteryx allows the display hypothesis to pass this test.