noordse dief

Literally, “nordic thief”, a blend of noordse stern (“arctic tern”, literally “nordic tern”) +‎ visdief (“common tern”, literally “fish-thief”). 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.

noordse dief m (plural noordse dieven, diminutive noords diefje n)

1. (birdwatching) A bird that is either a common tern (Sterna hirundo) or an arctic tern (Sterna paradisaea), but which cannot be determined with certainty.