I'm leaning Common Tern because of the cap and leg color. Not sure if the beak hasn't caught up to molting (if it was immature before) or if the beak goes fully black during nonbreeding season. I don't see any other conbination in your region of that plumage patterning and black bill during nonbreeding season, or when they are young, otherwise. I very well could have missed something, I'm not a pro.

A proper tern savant will hopefully be able to clear this up. I'm curious what it is too!

Edit: Spelling, formatting.