Hundreds of studies have searched for phenotypic traits that are associated with extra-pair mating success, as these traits might reflect the genetic quality or competitive ability of males.
Several factors may be contributing to the lack of consistent identification of traits that are sexually selected through extra-pair mating.
In birds, extra-pair mating can influence the strength of sexual selection, and paired comparisons between within-pair and extra-pair sires provide a powerful way to identify sexually selected traits that influence male fertilization success.
Extra-pair mating appears to produce comparable selection on body mass and feather parasitism of Tree Swallows in ON and WI.
Direct measurements of extra-pair mating behavior have not been published but the length of the breeding season and changes in testosterone suggest that extra-pair mating is common to all populations (Goymann et al.
2004a) suggested that high altitude Rufous-collared Sparrows likely have an extra-pair mating system and the sustained levels of testosterone may reflect a genetically polygynous population where males would be favored to have elevated concentrations of testosterone throughout the breeding season (Moore et al.
We emphasized that extra-pair mating systems result in increased male-male competition, but more than simply male/male aggression and territoriality.