A rare bird is a bird whose presence in a given geographic area is considered unusual. This can be due to natural factors such as gales or trajectory errors during migrations. The term erratic is sometimes used but it can be confusing, not always meaning the result of a change of areas of habitual residence following a migration error.
Organizations are specialized in the inventory of rare birds. The various European organizations have come together in the Association of European Rarities Committees. The latter was founded in 1993 on the island of Heligoland.