Juvenile-mimicry explains adult-juvenile resemblance in swallows and martins (Aves: Hirundinidae)

A similar phenotype exhibited by both adults and juveniles is often considered a self-evident default state due to shared genes and similar ecological niches, and thus the function of adult-juvenile resemblance is rarely addressed. An adaptive explanation for adult-juvenile resemblance is that adults mimic juveniles to attract mates by exploiting their parental care behavior and to avoid agonistic intrasexual combat from rivals that tolerate juveniles (i.e., the juvenile-mimicry hypothesis). Using a phylogenetic comparative approach, we tested the juvenile-mimicry hypothesis in aerial foragers, swallows and martins (Aves: Hirundinidae), in which adults and juveniles frequently encounter one another in their open habitat. We predicted that, if adults mimic juveniles, adult-juvenile resemblance should be enhanced in species with many young (i.e., a large number of models in relation to mimics) as well as species with a few young (i.e., a default state with limited intensity of sexual selection). This prediction was confirmed by a quadratic relationship between number of juveniles and adult-juvenile resemblance. In addition, as predicted under the juvenile-mimicry hypothesis, adult-juvenile resemblance was enhanced in species with multiple broods, in which juvenile-mimicry would be particularly effective due to the mating period followed by juvenile production. The observed pattern could not be explained by sexual selection for male ornamentation alone (i.e., with no juvenile-mimicry) even when considering the cost of ornamentation. An alternative explanation that juveniles mimic adults is also unlikely, as the situation favors the opposite pattern. The current study therefore supports the juvenile-mimicry hypothesis, indicating an adaptive function of adult-juvenile resemblance.