Extra-pair fertilizations and paternity defence in house martins, Delichon urbica

A study of the nestlings of house martins showed broods which had extra pair of chicks unrelated to their putative fathers. The production of the extra offsprings were attributed to extra-pair fertilization, where females copulate with males other than their mates to become fertilized with sperms that have better genetic quality. Extra-pair fertlization was also observed to take place only after the female starts to lay eggs because the males' guarding behavior becomes less aggressive during this period.