Swallowtail, of some sort

This butterfly kept fluttering around right before we headed home from our fishing yesterday. I figures soon as I got my camera it would leave, but decided to attempt capturing it anyway. I was pleasantly surprised when I got back to the sandbar to find it still fluttering from here to there. I just stood still near where it had been previously and it eventually came and posed for this photograph. I do not recall ever seeing one of these before...not in real life. I think I have seen one or two on other blogs.