It's hard to say exactly which species of bird is rarest, because numbers change quickly and in some cases the exact numbers are not known, but it's possible to identify which are the most extremely endangered birds.
According to the BBC list the rarest bird in the world may be the giant ibis. This large bird (over a metre tall) is the national bird of Cambodia but is now very hard to find as there are less than 230 pairs left and these are in danger from habitat destruction and eggs being taken by other animals.
The New Caledonian owlet nightjar may be even rarer - only 50 of these birds are still known (or presumed) to exist, though it is hoped there may be more.
Next is the enormous Californian condor with its 10 foot wingspan. This bird was very nearly extinct until recently, but it is hoped that a big conservation programme will rescue it.
Please see the list for details of the 100 rarest birds.