the bird has flown
Someone or something has left, fled, escaped, etc.; someone or something is no longer here. I'm afraid you're not going to find him here. The bird has flown.
bird has flown, the
The individual sought has gone away, as in Jean hoped to meet her editor at long last, but when she arrived the bird had flown. This idiom has been used for an escaped prisoner, and more generally, as in 1655 by William Gurnall ( The Christian in Complete Armour): "Man ... knows not his time ... he comes when the bird is flown." [Mid-1600s]
the bird has flown
If you say the bird has flown, you mean that the person you are looking for has escaped or disappeared. He'd been told to follow the woman to work and wait till she came out again. Instead he'd wandered off, come back at her normal leaving time and found the bird had flown.
the bird has flown
the person you are looking for has escaped or gone away.the bird has ˈflown
the person who was being chased or looked for has escaped or gone away: The police raided the house at dawn, but the bird had flown.