Occurrence and persistence of pseudo-tail spots in the barn swallow.

While numerous studies have confirmed sexual selection for ornamental traits in animals, it remains unclear about how animals exaggerate ornamentation across traits. I found that some Asian barn swallows possessed "pseudo-tail spots" on their undertail coverts adjacent to a well-known sexual signal, the white tail spots. A close inspection showed their remarkable resemblance, and, as a consequence, pseudo-tail spots appear to add white spots to the uniformly black central tail feathers, increasing the total number and area of white spots when spread tails are viewed from below.

Compensatory traits can explain the concave cost function of purely sexual traits.

The cost of ornamentation is often measured experimentally to study the relative importance of sexual and viability selection for ornamentation, but these experiments can lead to a misleading conclusion when compensatory trait is ignored. For example, a classic experiment on the outermost tail feathers in the barn swallow explains that the concave (or U-shaped) aerodynamic performance cost of the outermost tail feathers would be the evolutionary outcome through viability selection for optimal tail length, but this conclusion depends on the assumption that compensatory traits do not cause reduced performance. Using a simple "toy model" experiment, I demonstrated that ornamentation evolved purely though sexual selection can produce a concave cost function under the presence of compensatory traits, which was further reinforced by a simple mathematical model.