The passenger pigeon is the other end of the spectrum, an example of a species that was not thought to be threatened because it was always found in immense flocks; it seems, though, that passenger pigeons do not do well outside their gigantic flocks - collectively, as a species they may have required some advantage that being in a huge flock gave them. In any event, once a tipping point was reached the species disappeared entirely within a very short period of time, and they are extinct so we will never know for sure.
The California condor bottlenecked down to 27 individuals in captivity and 0 in the wild at one point; there are now estimated to be 435 living specimens, thanks largely to the tireless efforts of conservationists. As birds go, the very interesting Kakapo is another example. It is a flightless parrot - really peculiar bird, quite large, that long relied on insularity. It only inhabits a few rodent-free islands and doesn't do well at all in captivity; I fear we are going to lose the Kakapo forever, in not very long, despite the best efforts of conservationists.Source