Isn't the whole point of most Holy Grails, that they might be extinct, but there's a tiny chance of them still persisting somewhere? Jerdon's Courser has recently been refound in 1986 and again in 2009 after 86 and 33 (?) years without records, so I have no doubt that it's still out there
For species like Spix's Macaw or Javan Lapwing there have been recent (unconfirmed) records, while lots of potential habitat still needs to be surveyed for Pink-headed Duck or Himalayan Quail.
As for Black-headed Monarch and Silvery Pigeon: Both have recently been seen a handful of times and I might transfer them to the elusive species, but these records seemed so scattered that I believed that they still had the Holy Grail aura around them. Or did I miss some recent developments?
Sorry, have to disagree with some of your comments. First of all, if your definition of a holy grail bird is that it might be extinct, then few of the birds on your list qualify. They aren't extinct: they just aren't seen very often. Take Brown-banded Rail: if you are willing to spend a couple of months during migration looking at the bird trappers catch at Dalton Pass every night, there's a very good chance you will see one. It's not a holy grail (of Philippine birding, at least) because it might be extinct, but because almost nobody has seen it.
Same with Night Parrot - nobody who knew anything about the species seriously thought it was extinct. There were quite a few very credible sightings before it was actually photographed by John Young. I would say it's still the holy grail of Aussie birding.
Jerdon's Courser is an interesting one. It wasn't "refound" in 2009 after a gap of 33 years as you suggest - it was seen plenty of times in the intervening years by a reasonable number of birders (even me!). Sadly, it seems that sightings at the only known site have dried-up, and it hasn't been seen for several years. I am skeptical of the last claimed record - I believe it was made by a person trying to get funding to undertake a study, and I'm afraid I wouldn't take this at face value without further evidence. I would put Jerdon's Courser in the same boat as Slender-billed Curlew: there's a chance it might still exist, but show me the money.
Javan Lapwing: you're having a laugh! I take it you haven't actually been to Java? There's roughly the same chance this still exists as Reunion Solitaire. There are recent unconfirmed records of flying saucers too...
Pink-headed Duck - where is the "lots of potential habitat"? Most of its former range is densely-populated, intensively cultivated, and highly disturbed. I'm afraid that the days of large undisturbed areas of sub-Himalayan wetland are gone. People have looked quite hard and found no evidence that it's still there.
Himalayan Quail - sure it might still exist, but there's no evidence that it does. Mid-altitude parts of the Himalayas where it was known to occur are quite well-populated, quite heavily disturbed, and visited by many birders. I haven't heard of any credible claims, and I don't really agree that a bird's status should be based on what might be rather than what is known.
Boano Monarch and Silver Pigeon - the monarch is a relatively common endemic found on a small island off the coast of Seram. As Alan implied, you could go and see it next week if you wanted. Same for the Pigeon. What would make them more worthy of your list than e.g. Damar Flycatcher or Tanahjampea Monarch which are much more of a pain in the arse to see?
