noordse dief

Etymology
lit. 'nordic thief', a. These two tern species resemble each other a great deal visually and distinguishing features based on which an exact determination can be made are often difficult to spot from a distance in the field, especially by casual observers. This has led some birders to refer to birds they observe of these species using this more general term instead of attempting an exact determination.

Noun

 * 1)   A bird that is either a common tern  or an arctic tern, but which cannot be determined with certainty.