Revisiting the evidence for inbreeding avoidance in zebra finches

Mating between close relatives nearly always leads to inbreeding depression, which should promote the evolution of inbreeding avoidance mechanisms, especially in sexually monogamous species. The zebra finch, a predominantly monogamous species, has been shown to suffer strongly from inbreeding depression, and this species has been used repeatedly in studies of inbreeding avoidance. However, no conclusive evidence for such avoidance has emerged. Here, we summarize the previous research in a small meta-analysis showing that zebra finches appear to mate randomly with regard to relatedness (odds ratio for full-sibling mating = 0.96). Nevertheless, we notice that kin recognition by direct familiarization might have been prevented in all the previous studies because siblings had always been experimentally separated before puberty and only reunited during adulthood. Recognizing individuals across different life stages may be cognitively demanding; therefore, in this species, it may require that siblings stay in contact throughout development. We conducted an experiment where birds were given the choice between a full sibling that stayed with them without interruption from hatching until adulthood and an unrelated bird familiar from independence (35 days of age) to adulthood. In contrast to all earlier studies, we found a significant avoidance of inbreeding (odds ratio = 0.50). Although other mechanisms cannot be excluded, we suggest that zebra finches avoid inbreeding only if birds can keep track of their kin, and we discuss implications for the design of follow-up studies.