I think it's a Roseate Tern, all the features of this bird match with the description, except of the length of its tail streamers.
Olsen KM and Larsson H (1995)* mentioned the Roseate Tern (1st Summer) is similar to very pale Common Tern of similar age, but note dark legs and longer outer tail feathers. Unfortunately, they do not have the information of the 2nd summer plumage, but presumably the plumage would be very similar to adult summer.
Comparing the bill shape and length of the Common Tern and Roseate Tern, the bill of Roseate Tern looks longer and thinner.
*Olsen KM and Larsson H (1995) Terns of Europe and north America, London : Christopher Helm ( Publishers) Limited.
