Female barn swallows select mates with symmetrical wings and tails and prefer longer tail feathers, all traits connected with increased strength, vitality, and longevity, as well as greater disease resistance.
Several studies have researched sexual selection in barn swallows.
Males try to attract females by spreading their tails to display them and singing.
Female barn swallows have been documented selecting for symmetrical wings and tails in potential mates. Males exhibiting greater symmetry acquired mates more quickly than did asymmetric males.
Asymmetry can result from genetic factors such as inbreeding or mutations as well as from environmental stress such as food deficiency, parasite infestation, or the presence of pathogens. Asymmetry of physical characteristics in barn swallows tends to be transmitted to the young in distinct parent to offspring patterns. Tail asymmetry tends to pass from father to son and from mother to daughter. Alternatively, wing asymmetry does not appear to transfer at all on a reliable basis from parent to offspring. Individuals affected by these factors not only exhibited asymmetry, but also decreased strength and longevity. Therefore, females that selected symmetrical mates would presumably be selecting superior mates.
In addition to selecting for symmetry, females also tend to select males with longer tail feathers. A connection between the tail length of male barn swallows and their offspring’s vitality and longevity has been observed. Males with longer tail feathers exhibit traits of greater longevity which is passed on to their offspring. Females thus gain an indirect fitness benefit from this form of selection, as longer tail feathers indicate a genetically stronger individual who will produce offspring with enhanced vitality. Individuals with longer tails have also been observed to demonstrate greater disease resistance than their short-tailed counterparts. There is also evidence that males select female mates with long tails.