When you get too close they fly away
My goodness. Certainly nesting birds need to be protected, but very often I approached feeding birds, they notice me and fly away. I carefully watch behavior as I approach, and if it looks like they notice me, I back off. However, on countless occasions I get noticed, and of course this is not my plan, but they fly off anyway. My attitude is when you have violated their comfort zone, they fly off, and I can't imagine this unduly stresses the birds. If I have taken pictures, and I'm finished, I slowly back away. Now if the rule was to not violating comfort zones, which sounds like political correctness to animals, I immediately need to get rid of my camera equipment. Sorry, I don't plan on it.
You want to talk about stress? I would put audio recording to attract birds at the top. See what happens when you attract two males of a species, responding to a call, together, and see the fight that ensues. Now we are talking life/death struggles and real stress! regards~Bill