Before migration was understood, people struggled to explain the sudden annual disappearance of birds like the white stork and barn swallow. Besides migration, some theories of the time held that they turned into other kinds of birds, mice, or hibernated underwater during the winter, and such theories were even propagated by zoologists of the time.[6][7][8] The Rostocker Pfeilstorch in particular proved that birds migrate long distances to wintering grounds.[9]