Complex sentence: The Royal Tern is not usually found in Europe but it has been seen in the western European countries of Spain and Gibraltar, but it has been seen farther north, but those cases are not confermed.
Split into simpler sentences: The Royal Tern is not usually found in Europe although it has been seen in the western European countries of Spain and Gibraltar. Also, it has been seen farther north in Europe, but those cases are not confermed.