Plumage coloration, not length or symmetry of tail-streamers, is a sexually selected trait in North American barn swallows

Sexual adornments often vary markedly across a species' range,which presumably is owing to differences in local environmentalconditions and the associated selection pressures, such as naturalversus sexual selection or the relative signaling value of differentornamental traits. However, there are only a few reported examplesin which the information content of mating signals varies geographically,and even fewer in which a set of secondary sexual traits servesdifferent signaling functions in different populations. Classicstudies of sexual selection in the European barn swallow (Hirundorustica rustica) demonstrate that elongate tail-streamers provideseveral reproductive advantages to males and females and areused as reliable signals of mate quality. Here, we show thattail-streamers do not appear to confer these same benefits ina population of barn swallows from North America (Hirundo rusticaerythrogaster). Instead, ventral plumage coloration, which ismore exaggerated in North American swallows compared with theirEuropean counterparts, predicts patterns of assortative matingand annual reproductive success in H. r. erythrogaster. Theseobservations support the idea that ornamental traits can servedifferent functions among animal populations and suggest thatgeographic variation in different sexual signals may facilitatepopulation divergence, which may ultimately lead to speciation.