In this question, there are no options given to choose from. So i would answer this question based on my knowledge. The most likely cause of the observed variation is adaptation and evolution. The birds of that island adapted to the food source and then they evolved as per the requirement of the place and food source.
To find out if it's true or not
medial cuneiform
It lies on the inner side of the foot. It lies in the front of navicular bone and behind the metatarsal.
