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 beyond their original traits. I found that some Asian barn swallows Hirundo rustica gutturalis possessed pseudo-tail spots in their undertail coverts. A close inspection showed its remarkable resemblance to the white tail spots, a well-known sexual signal in this species, although pseudo-tail spots in the undertail covers do not incur any flight cost, unlike the white tail spots on the tail itself. Presence of pseudo-tail spots can thus represent an initial stage of a deceptive elaboration as predicted by sexual selection theory. 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.