It's probably scoliosis. Click here to see source Abstract Summary Scoliosis is a genetic defect occurring in the white Pekin duck. It results from an abnormal growth of individual vertebrae. Curvature of the spine is not obvious at the time of hatching, but histological evidence of portions of the notochord is present and this is associated with an abnormal growth of the vertebrae. Scoliosis may be grossly obvious in ducks 40 days old. Of 282 white Pekins, 28.4% had scoliosis but only a few had a vertebral abnormality that was evident in the living bird. Scoliosis was not observed in the Mallard or the Muscovy, but occurred in 22.2% of 27 hybrids resulting from the mating of the Muscovy male with the white Pekin female.