"blink, The Power of Thinking without Thinking" by Malcolm Gladwell is a book of the moment. The base idea of the book is that many of our initial instincts are better than our considered thoughts. It's a compelling book, but early on he says this;
"The ornithologist Dave Sibley says that in Cap May he once spotted a bird in flight from two hundred yards away and knew instantly that it was a Ruff ... He had never seen a Ruff in flight before, nor was the moment long enough for him to make a careful identification, but he was able to capture what bird-watchers call the birds "giss" - its essence - and that was enough".
David Sibley is later quoted as saying "you know what it is at a glance".
Well ... yes and no. If you watch enough birds you can pick up that instinct. But in my experience the above is a recipe for mistakes. There are all sorts of circumstances when birds can depart from their normal behaviour; moulting affects the flight and general appearance, wind causes birds to fly differently, and I've found watching sea-birds in inland or quiet conditions completely useless for watching seabirds migrating. Birds on migration find themselves in strange places and behave in strange ways. And overseas, the light is different, birds behave differently, and they look completely different.
So yes, an instinct is useful, but most birders then get down to the serious business of watching properly and going logically and thoughtfully through the features until they are confident of the ID. This is the best way to do it, and its completely the opposite of what Malclom Gladwell says in his book.