Birds or any animal for that matter live in a situation we really have no comprehension of, intellectually maybe but as matter of course most certainly not.
And that is the predator/prey relationship they have on a daily basis. They are either food or want to acquire another for food or both.
And having this thought in the back of your mind when trying to capture images of these creatures should be there, are you presenting yourself as a predator or prey or a non-threatening species.
Dressing in camo and trying to sneak up on an animal while it may make you immediately less obvious, to humans mostly, most species are aware you are there. Those that seemingly are not aware have either recognized you as a non-threat or literally have no fear of humans or large moving shrubberies. 
Camoflague is most effective when used in a single position, sitting and awaiting a subject, it can make you less obvious but many many species will know you are there already either by sight, sound or smell. These animals know their territories and know when a new object has plopped in their area. Raptors will pick you up much earlier than you can image
They also recognize changes in the "usual" they encounter, vehicles for example. I often shoot at a favorite refuge from the car and have found while the car in and of it self doesn't spook the subjects, a change in the vehicles profile does. That is if I pull up and then run thelens outthe window, or an arm or a head, the subjects will pull back to distance they feel more secure at.
However if I pull up with the lens, head, arm or whatever already out of the vehicle it is not as threatening and the images can be captured. Be ready before you are close and you can get closer.
Being stealthy or sneaky can be preceived as behavior of a predator and the subjects will react very much the same as if a predator were really there, you see this in your backyard where the birds would normally sit through just about whatever you might be doing but if you suddenly appear in the yard everything takes flight and the risk assessment is then done by the birds. Often very quickly they are back to feeding or doingtheir bird stuff after recognizing you as a non-threat.
These animals are very much aware or their surroundings and haunts, a good friend attempted to photograph some Kingfishers and went out and setup his blind and then spent the next two weeks on an almost daily basis trying to capture shots of them, whether he went in before light or even spent the night in the blind he could not capture an image of these two birds except as they flew by rattling their alarm call.
After we talked about this it was realized he had set his blind in a perfect position to shoot a particular perch but had placed the blind directly under another favored position and they "recognized" the change in the area. Much like if I decided to photograph some one in their living room by hiding partially behind the couch and partially under the drapes, a change in the room would be noticed. And I probably wouldn't be invited back for dinner.
Understanding the subjects habits will bring more success than just finding the "perfect" spot, he had noted the subjects mate would use the location where he was at on occasion but didn't take that into account and so had a couple week learning experience.
The whole point is to understand the subjects "world view" if you will, if you act as if you are a predator then prey will leave the area, if you act as prey you maybe eaten (which could make for some good images however have a lens handy with a short min focus distance so the images will be in focus.) 
And knowing the subjects habits, haunts and times they perfer to do certain things like feed, groom, rest or travel. These are usually quite regular for just about every species, things change a bit when there is young to care for but otherwise they operate much like we do on a schedule.
Their schedule though is based on different clock than ours, they use the Sun's rising and setting which changes on a daily basis, getting earlier or later based on the season. If you find an animal some place 2 hours after sunrise and go back a month or two later at the time you saw on your watch and spent a few days you wouldn't probably capture the similar behavior. Because the time of the sun rise has changed.
But if returned at the same two hours after sunrise you would more than likely do so, a lot of species behavior is not based on a daily set of rounds but maybe every couple of days or every week,. A coyote I often photographed Icould count on passing a certain place about every 3 days. These animals range a lot but do have hime ranges and will cover all of it. It just depends on the scale and demands they animal currently has on it.
There is a bit more I could go on about like movement, crawling is often not seen as threat where walking towards the subject is, eye contact or lens contact, ever stand back and look into the lens as seen the animal? With the eyepiece un-covered it can look like a giant eye complete with pupil as light is gathered through the viewfinder.
And some more but the Sun is up and time to go play. Hope this helps a bit.
Bottomline know your subject, don't be preceived as threat and be patient.
