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

ABSTRACT 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.