I work on the motto that the birds are smarter than I am. They always know I am there before I know they are there, let alone before I can spot, position scope, whip the camera off, focus, replace camera, frame and shoot.
They always know you are there. The trick is getting them not to mind that you are there. Just being in the one place for a decent amount of time without moving much helps a lot.
This is particularly noticable with waterbirds and waders, as they are often found in places where there simply isnt any cover at all. All you can do is make your way to a spot you like and then stand there waiting. Many species will gradually return to forage around you. On the other hand, some won't. In particular, some species seem to be influenced by other species around them. For example, last summer I drove quite some distance to see the rare Freckled Duck. They were at a sewage pond, no cover at all: just a big square pond cleared around for 30 or 40 metres in all directions. Lacking options, I just walked slowly down close to the water's edge and waited.
It was 47 degrees that day: seriously hot, but I stood, more or less without moving, for a half hour or so, waiting. (I'd have waited considerably longer in sensible weather, but hat or no hat, standing in the sun when it's 47 degrees in the shade (yes, that's the official BOM figure) is hard work. Little by little the stilts and avocets and dotterels came back, the Pacific Black Ducks drifted closer. But the Freckled Ducks stayed obstinately at the other side of the pond. I'm virtually certain that the only reason they did this was because they were taking their cue from the Pink-eared Ducks.
Bloody Pink-eared Ducks! They do that to me all the time: everyone is cool: I'm only a human and I'm just part of the landscape. Life goes on. There are birds everywhere. But then the Pink-eared Ducks spook, and every other species witin earshot takes off too. Wham-Bam! Empty pond.
Maybe I should dress up as a Pink-eared Duck.
I have a friend who says that the key spook-factor is the human outline; that if you can break up the head-and-shoulder outline, you can get much closer to all sorts of creatures. Assuming he is right, what can you do to break up your outline? And how silly does it make you look? No matter: if it gets me closer to the birds, I don't care.