As most of us with an interest in extinctions know, the last dusky seaside sparrow died a year or so ago. It was one of those rare cases of a small species endemic to a given limited habitat and closely related to a larger, more widespread species.
With that bird no longer extant, and the population of California condors slowly climbing out of the danger zone, what is now thought to be the rarest living bird?
How about the kakapo?
The cahow seems to be somewhere in between the California Condor and the kakapo in terms of numbers.
Another New Zealand bird, the takahe, would be one of the rarest I suspect.
The rarest known bird is the Poo-uli of Hawaii. The most recent counts recorded only three birds.
However, I heard last night that a recent search failed to find any, so perhaps we may need to look for a new candidate.
Ah, yes, New Zealand – the place where all the indigenous mammals fly, and most of the indigenous birds don’t! 
Thanks for the answers, LL and Cunctator!
I had the great privilege of working for the New Zealand Wildlife Service (now the NZ Department of Conservation) from 1981-1985, and participated in the Kakapo recovery program. (I calculated I handled 10% of the known world population of the species at that time.) While it was really touch and go back then, they have started a comeback and are now up to a whopping 86 individuals.
I also had the chance to work a little with Takahe. Although they have been in decline, there are still 130 in their natural range, plus 60 on offfshore islands and others in captivity.
Never let it be said that New Zealand can outdo Australia in any respect. Here’s a list of Australian birds considered to be critically endangered.
More on the Po’o-uli. The last bird in captivity just recently died, two others in the wild have been missing for more than a year.
A rarer species than the Kakapo, Takahe, or Cahow is probably the Alagoas Currassow, now extinct in the wild. There were 44 in captivity in 2000, the most recent date I was able to find data from.
The rarest currently living bird species is probably one that’s thought likely to be extinct, but of which a few individuals still survive in some remote location (as was the case for the Takahe).
North American candidates would include Bachmans Warbler and the Eskimo Curlew. Some still hold faint hope for the Ivory-Billed Woodpecker.
As I understand, while the possible population at the old “last stand” of the IBWP in Louisiana is subject to a lot of skepticism, there have been reliable sightings and/or confirmed calls at a reported population site in Cuba, but they haven’t been validated by professional ornithologists and published. If you’ll excuse the Princess Bride-ism, they’re only mostly extinct!
For that matter the rarest currently living bird species is probably something that has never been officially identified…something in a remote, restricted habitat somewhere which is quickly being eliminated.
I therefore took the OP to mean species for which there was very recent evidence (within the last year, like dusky seaside sparrow example mentioned in the OP) they weren’t extinct.
Sad as it makes me I believe the Eskimo Curlew and Ivory Billed Woodpecker are probably gone for good. Bachman’s Warbler I still hold out hope for.
That was my intent – not the possibility that there’s a Heath Hen or a Carolina Paroquet lurking somewhere, but “We’ve sighted them relatively recently, so they are in fact still around as of ___.”
The Ivory-bill was confirmed to still exist in Cuba in the 1980s, by professional ornithologists, but the last record from the main known site was in 1988. However, there does seem to have been a new record from another site as recently as 1998.
And this one, Colibri:
**The Official Ivory-bill Is Alive Thread ** in MPSIMS
Although probably not as badly off as the Ivory-billed woodpecker, Spix’s macaw is pretty severely endangered, with a total world population of only about 60 birds (up from a low of 11 courtesy of captive breeding). There’s only one known Spix’s macaw still living free in the wild.
(Unfortunately the list of critically endangered birds is a long one.)
One suspects it has been ever thus.
(And can we stipulate that this does not mean I’m in favor of or indifferent to threatened extinctions.)
Probably not. The extinction rate for birds went up drastically after humans came on the scene.
Good news! Cases of unextinctintion are also on the rise, though!