“That bird has flown” is said when someone goes looking for a particular person and finds they have already got away. It usually carries a note of frustration or wry resignation, as when police arrive to make an arrest only to find the suspect gone, or when a search for someone turns up an empty room. The phrase treats the missing person as a bird that has simply taken off, leaving the pursuer with nothing to show for the effort. It is most often used of someone actively being sought or chased, rather than of an ordinary absence, and whoever discovers it is rarely pleased.
What’s the origin of the phrase ‘that bird has flown’?
The phrase is old, and its first clearly dated use points to the mid-1600s and to the language of escaped prisoners. William Gurnall’s 1655 religious work The Christian in Complete Armour uses the idiom in that sense, applying it to someone who has slipped out of custody or out of reach. From there the image spread to its broader modern use, covering anyone who has gone before they could be found or caught, not just literal escapees.
The underlying picture needs no special explanation: a bird that was sitting in a cage, a tree or a hand has simply flown off, and whoever wanted to catch it is left looking at an empty space. A claim circulating online traces the phrase specifically to falconry, the idea being that a trained hawk or falcon has slipped its handler and taken flight. That account appears only on a single idiom-aggregator site, is not sourced to any dictionary or historical reference, and has not been corroborated elsewhere, so it is best treated as unverified rather than repeated as fact. The plainer and better supported story is simply that the phrase draws on the general behaviour of birds taking flight, first attached to the specific case of an escaped prisoner and later widened to anyone who cannot now be found. Dictionaries today define it in exactly that broadened sense, used whenever the person being looked for has already gone.