It has been suggested that a high degree of relatedness could explain the occurrence of alloparental care in birds, but few studies have confirmed if there is a genetic relationship between foster parents and chicks. Using one case of adoption involving allofeeding and allodefense of a South Island Robin (Petroica australis) fledgling by a neighboring male, we assessed whether the adoption was the result of close kinship, care directed to offspring from an extra-pair copulation, or altruistic behavior toward non-kin. Our genetic analysis showed that the foster father was not the adopted fledgling's biological father, nor was he closely related to the chick. We conclude that his altruistic behavior may have been accidental, driven by a breeding male encountering the begging of an unrelated chick that had moved into his territory. The low level of adoption in this population (1.5% of 65 chicks) also supports the hypothesis that alloparental care in South Island Robins is likely to be the result of errors in kin recognition.