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. Pseudo-tail spots on the undertail covers do not incur any flight cost, unlike the white tail spots on the tail itself, and thus presence of pseudo-tail spots can represent an initial stage of a deceptive elaboration as predicted by sexual selection theory (i.e., males can elaborate traits with no additional flight cost, uncoupling flight cost and trait expression). The frequency of pseudo-tail spots in the study population remained low even a decade after the first observation (ca. 7%), but was higher compared to other populations (e.g., 1% in another Japanese population). The slow progress of evolution, perhaps due to the low detectability of the trait, provides a unique opportunity to observe contemporary evolution of ornament exaggeration across traits. Further research with wider spatial and temporal coverage is needed to better understand the evolutionary and ecological importance of the trait.