section_path: Reproductive systems/Outcrossing
section_name: Outcrossing
paragraph_text: Outcrossing
  
  Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny (see heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (see inbreeding avoidance).
  
  Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit (Parus major) when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.
  
  Purple-crowned fairywren females paired with related males may undertake extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints.
  
  Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates. Although both genders disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated.
  
  Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin. Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups.