Genetic & Developmental Defects or Injuries
Sanderling
This individual probably has a developmental defect as its left leg is turned around 180 degrees and the foot is likewise useless. It hops around perfectly well on just one leg and manages quite well finding food buried in the sand. These pictures are proof it made it successfully to adulthood.
Sanderling-8744

26-Aug-2016 Chincoteague National Wildlife Refuge, VA