A costly, cumbersome tail can be exactly what proves a peacock is fit

A heavy, drag-inducing tail should make a peacock easier for predators to catch, so why would evolution favour it? Amotz Zahavi proposed that the very costliness is the point: only a genuinely strong, healthy male can afford to carry such a handicap and survive, so the ornament works as an honest, hard-to-fake signal of quality that cheaper displays couldn't provide.

— Amotz Zahavi, Mate Selection: A Selection for a Handicap — Journal of Theoretical Biology, 1975