I have been birding for many years, and am still learning. Thankfully, there will never come a day when I know everything there is to know about birds. What fun would there be in that?
I find that you can struggle with identifying a bird for years and then one thing will strike me and afterwards I'll be able to identify the bird on sight. Sometimes, these identification hints come from another birder, a magazine article, a blog post, or a field guide (that's why most birders have multiple field guides--the identification information varies).
I used to struggle with the black vulture on the wing. I don't know why as their shape is quite different from the turkey vulture, but there you have it. But then somewhere I read about a triangular "window" at the wing tips, and I haven't had a problem since. And with that key ID information, I have been able to spend more time looking at the bird, just to watch. Even more differences then become apparent when you're not so focused on trying to compare ID points.
This "window" is very helpful when driving. Friday, I was driving down the highway at 70 mph and caught sight of a flock of black birds over a field. It's not a good idea to spend a long time scoping out the birds at that speed, but in this case, a fraction of a second was all that was required--the field mark was obvious. There are no turkey vultures in this area in the winter, but when I initially caught sight of the flock, I thought "crow." Size-wise that doesn't make much sense, but at a distance and at speed (and supposedly focused on traffic), it's easy to jump to the wrong conclusion.
To a birder experience with the black vulture, it might seem this was an easy ID, and it was. But I am often asked by new birders how I can ID birds so quickly. All I can tell them is practice, practice, practice. Every year another bird goes from my frustrating list to my familiar list. I'm not likely to live long enough to move all of the birds from one list to another, but I look forward to the process.