Unknown Seabird.

Today I went to Bristol Zoo with my 3 1/2 year old Grandson.Even @ his age,he knows all the animals in the zoo.And where they are? I had a real education from him!   lol
  Anyway this is not in my Bird book so I can only assume it is not British.It flies in a flock in an open Aviary,so must be quite hardy?
 As you can see,it is beautifully marked.In flight it resembles a Tern and is very agile.
 Any one any idea?