Unusual Bird: Problem swallowing, possible deformed mandible

It is very unusual to have a robin appear in my yard in December. It first arrived on December 22 and continues to be present on and off throughout the day since then. It seems to have trouble swallowing – opens and closes its mouth constantly and body moves as if it has hiccups when eating. Starts off by eating berries and then flies to the ground for ground seed. When its bill is closed, it seems to look fine, but when it opens its mouth, there seems to be a bill problem as if part of the bill as broken away.