Adoption: Adaptation or Reproductive Error in Eastern Bluebirds?

We describe an adoption event by a female Eastern Bluebird (Sialia sialis) and suggest a mechanism by which adoption could be adaptive. The foster female adopted three young and we were able to quantify two measures of parental care (nestling provisioning and nest defense). The foster female successfully reproduced with the father of the young in the same location later in the breeding season. The adopting female may be increasing her fitness by gaining future reproduction with a fertile mate at a productive nest box through parental care for young that are not related.