Original Paper

Behavioral Ecology and Sociobiology

, Volume 61, Issue 11, pp 1725-1733

First online:

Female tree swallows (Tachycineta bicolor) increase offspring heterozygosity through extrapair mating

  • Mary K. StapletonAffiliated withDepartment of Biology, Queen’s University Email author 
  • , Oddmund KlevenAffiliated withDepartment of Zoology, Natural History Museum, University of Oslo
  • , Jan T. LifjeldAffiliated withDepartment of Zoology, Natural History Museum, University of Oslo
  • , Raleigh J. RobertsonAffiliated withDepartment of Biology, Queen’s University

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Recent attention has focused on genetic compatibility as an adaptive function for why females engage in extrapair mating. We tested the genetic compatibility hypothesis in tree swallows (Tachycineta bicolor) over five breeding seasons using data from ten microsatellite loci. Tree swallows are socially monogamous passerines exhibiting high levels of extrapair paternity. Overall, we found that 47% of offspring were the result of extrapair fertilizations, and 83% of females produced at least one extrapair offspring. Consistently for all years, extrapair offspring were more heterozygous than their maternal half-siblings, which is in accordance with the genetic compatibility hypothesis. The difference was largely caused by the high heterozygosity of extrapair offspring sired by unknown males, suggesting that females are engaging in extrapair copulations with geographically distant males to increase the likelihood of being inseminated by a more compatible mate. Our findings support the idea that postcopulatory mechanisms are important for females when assessing potential sires for their offspring.


Tree swallows Genetic similarity Extrapair mating Extrapair paternity Tachycineta bicolor Heterozygosity Genetic compatibility Genetic diversity Microsatellites