It's just a flesh wound.
I saw this bird today in south central Pennsylvania. There were Forster's and Common Terns there, but this guy stayed with the Caspian Terns. It was also not plunge diving, but swooping down to pick up prey off the surface of the water. Can a first year Common Tern still have orange legs? Or is it a Least Tern. I didn't notice it was banded until looking through my pictures.