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Sexual selection is the selection of certain traits based on sexual preference by one of the sexes. This is typically seen as female choice driving the evolution of ornate plumage and elaborate songs used by males during courtship. Sexually selected traits are often very costly for males and can lower survivorship, however they increase fitness. Traits can be costly due to the energy required to have them, or they can attract predators. Brightly colored feathers can be a sexually selected trait that lowers survivorship because it makes the bird more peceptible to preadators. However, colorful wings increases fitness because females are attracted to colorful wings so the male is more likely to attract mates and therefore produce more offspring. In some cases, sexual selection can result in "run-away" sexual selection in which the males evolve very elaborate forms of ornamentation that keep evolving as female preference evolves to prefer more elaborate traits. An example of this is the tail feathers of peacocks. Peacocks that have a larger surface area of eye spots have more young that grow to reproductive age, so the fitness of peacocks increases with the number of eye spots they have on their tails. This provides evidence to support the hypothesis that female peacocks have a sexual preference towards males with many eye spots, indicating high ornamentation increases fitness.