Extra-pair fertilizations in mountain white-crowned sparrows

Summary A population of white-crowned sparrows (Zonotrichia leucophrys oriantha) located near Tioga Pass, in the central Sierra Nevada of California, was studied during the 1985 and 1986 breeding seasons to determine how often chicks were not the offspring of both adults who reared them. Blood samples were obtained from 35 nesting pairs and feather pulp specimens were collected from all 110 young they fledged. Electrophoretic analyses of 4 polymorphic loci revealed that a minimum of 9 nests (26%) contained a chick that could not have been conceived in a mating between the attending pair, and that at least 15 chicks (14%) were not offspring of both attending adults. Using two recently developed probability indices, it was estimated that 34%–38% of chicks hatched at the study site were not offspring of one of their putative parents. Because there was no evidence that intraspecific brood parasitism occurred, all the mismatched young apparently resulted from extrapair copulations. Our data, along with similar results from other species, raise new questions about sexual selection and patterns of parental care in monogamous birds.