‘In accordance with their inherited calendars, birds get an urge to move.? When migratory birds are held in captivity, they hop about, flutter their wings and flit from perch to perch just as birds of the same species are migrating in the wild.? The caged birds ‘know’ they should be travelling too.? This migratory restlessness, or Zugunruhe, was first described by Johann Andreas Naumann…[who] interpreted Zugunruhe to be an expression of the migratory instinct in birds.

William Fiennes, ‘The Snow Geese’