the person you are looking for has escaped or gone away.
a bird in hand
something that you have securely or are sure of.
This phrase refers to the proverb a bird in hand is worth two in the bush, current in English since the mid 15th century.
a bird of passage
someone who is always moving on
Literally, a bird of passage is a migrant bird.
a bird's-eye view
a general view from above
the birds and the bees
basic facts about sex and reproduction as told to a child
birds of a feather
people with similar tastes and interests
This phrase comes from the proverb birds of a feather flock together which has been current in this form since the late 16th century. Its origins may ultimately lie in the Apocrypha : the birds will resort unto their like (Ecclesiasticus 27 : 9).
serve a prison sentence
In this phrase bird comes from rhyming slang birdlime time.
a person who gets up, arrives or acts before the usual or expected time
This expression comes from the saying the early bird catches the worm, meaning that the person who takes the earliest opportunity to do something will gain an advantage over others.
flip someone the bird
stick your middle finger up at someone as a sign of contempt or anger
1994 - Washington Post Magazine - We could simultaneously honour America, break the law and flip the bird to all the do-gooders.
give someone the bird = get the bird
boo or jeer at someone (or be booed or jeered at)
This phrase first appeared in early 19th century theatrical slang as the big bird, meaning a goose. This was because the hissing of geese could be compared to the audience's hissing at an act or actor of which it disapproved.
have a bird
be very shocked or agitated
1992 - Globe & Mail (Toronto) - The Washington press corps would have a bird if the president to be appointed his wife to a real job.
kill two birds with one stone
achieve two aims at once
a little bird told me
used as a teasing way of saying that you do not intend to divulge how you came to know something
strictly for the birds
not worth consideration, unimportant
This expression was originally US army slang. It may be an allusion to the way in which birds eat the droppings of horses and cattle.