Birds will almost always exhibit a certain amount of concern when I first enter an area, but I give them a chance to adjust. Usually it's only a matter of five to ten minutes before they will go back to their routine activities—after deciding I'm not a threat.
I've photographed around 180 bird species and have found that the color of my field clothing has made very little difference in how birds react to my presence. More significant is how I behave around them. A birder dressed in jungle fatigues walking into an area where birds are present will be noticed just as much as one wearing a white t-shirt. In fact, a stealthy-looking camouflaged human figure that appears to be stalking might pose an even greater sense of threat to a particularly leery bird that has no appreciation of how badly you want to photograph it.
Regardless of what color you're wearing, most birds will adjust to your presence so long as you sit or stand still. If you have to move, do so with smooth, deliberate motion. If I need to reach for an adapter or fresh camera battery or lift my binoculars to check out what birds are in the area, I try to do so with as little jerkiness as possible. And be silent—birds will invariably react to a sudden shift in position or any sharp sound.
Few people have observed me digiscoping because this process requires silence and a minimum number of objects or potential threats for the bird to assess and concern itself with. I know this may sound a little counterintuitive, but when I'm super close to a bird, I avoid making eye contact with it as I photograph. I'll act as if I have no interest whatsoever. However, when a bird has a problem with my presence, I slowly back away, leave the area, and try a different location.
Article and Yellow-rumped Warbler photo contributed by Mike McDowell, an avid digiscoper, amateur naturalist, and Eagle Optics employee. Visit Mike's Birding and Digiscoping Blog.