Polymelia and Syndactyly in a Swainson's Hawk (Buteo swainsoni).

A hatch-year Swainson's Hawk (Buteo swainsoni) recovered from Modoc County, California, US, on 12 August 2012 had malformations of the rear limbs consisting of bilateral polymelia and syndactyly. We describe the malformations and evaluate potential causes. Postmortem examination revealed varus rotation of both femurs and abnormal appendages originating from the distal medial surface of the tibiotarsi with two nonfunctional digits on the right leg and one digit on the left leg. There was syndactyly between digits III and IV of both feet. Avian pox viral dermatitis was present on the skin of the ventral abdomen. A definitive cause of the skeletal malformations was not identified.