Hi, is this a young Forsters Tern?
CLOSED: Forsters Tern?
Well Okay, I'll get this figured out. Thank you - new bird either way
Yep, Caspian Tern; the key points are the big heavy bill (like a carrot!) and large size (twice the size of Forster's), and also when in winter plumage, a more diffuse black head cover, unlike Forster's white head with a sharply defined black eye patch.
Thank you everyone - I got the difference now, it's finding out those key factors that are so helpful. The book I'm using did not explain it well.