I often look at things from a different perspective than others.
For example, while driving somewhere I may notice some birds fly across the road, from left to right. Then just seconds later, other birds are flying across the road from right to left.
I ask myself, "How do birds make a decision that they want to be on the other side of the road?" Some birds only fly short distances and they almost always can see their future landing location prior to take off. However, others like Finches, often fly distances such that their destination was not visible from their launch point. So why did they leave Point A and fly to Point B? You can't use a food argument when the landing point is far from the launch point.
In fact, if you broaden the look, you will realize that birds make hundreds of decisions every day. If you watch birds such as Kinglets, Warblers, Woodpeckers forage, you can follow their train of thought a little. They stop for a few seconds, look around, and hop to a new location presumably based upon what they have seen during the looking. Such decisions are made with food in mind.
A major activity for all birds is "threat assessment". Even Bald Eagles become timid in the presence of humans. Threat Assessment activity must in the main, be learned.
I have watched Shorebirds and Ducks at Burnaby Lake respond to an overflight by a Northern Harrier. They all took the same action. They went into the water. However, when a Bald Eagle overflies Ducks, they take to the air.
So they have made a threat assessment and take actions based upon what the threat is. All birds seem to be self aware. That is, they know when you are watching them. A simple example occurred when I spotted a Sharp-shinned Hawk sitting by a trail at Burnaby Lake. Joggers and walkers passed his position, but none spotted the Hawk and he remained there unaffected by their nearness. However, when he realized that I had spotted him, he flew off when I was 10X the distance that the walkers and joggers were.
Recently I have been more successful at spotting small birds sitting unmoving in bushes and trees. Previously I would key on motion, but now have added shape to my visual cues. I have seen the same response whereby walkers don't disturb the birds, but when they realize that I have spotted them, I see them perk up, then flee down into hiding.
Although I no longer follow THOSE THREADS on these forums, I imagine that there has been much recent discussion regarding "stress" to birds caused by photographers and aggressive watchers.
Here's a simple example. I was watching a nearby Purple Finch who was posing nicely on a branch in the sunlight. He was obviously aware of me, but was facing me, not facing away in a flight-ready posture. I snapped a few photos, then caught a glimpse of motion, looked up and saw a Cooper's Hawk land at the top of a tree, perhaps 20 meters in elevation. I was about 5 meters from the Finch. He immediately turned his head upwards and studied the Hawk for a few seconds. He completed his threat assessment and flew off into the thicker undergrowth out of my sight. So how "stressed" was he by my presence?
Here's another example. I was walking along the dyke at Boundary Bay carrying my big lens. I noticed about 100 meters ahead of me, lots of bird activity on or near the path. There were more than a dozen Robins on the ground and even one Flicker amongst them.
I approached to about 20 meters from them and stopped. I stopped because I noticed that right beside me was a shrub with lots of those red berries on it. There was a flock of Waxwings in the shrub and also many Finches and Sparrows were in the shrub too.
I started to examine the Waxwings, looking for yellow in the wings of any of them. Meanwhile I noticed that the Robins were mostly ignoring me. The Flicker, however, lifted his head every few seconds to check my position. A suspicious bird apparently. Then I started to check the Finches to see if any might be Redpolls among the House Finches. Basically I was in the midst of many birds - perhaps as many as fifty. I was tolerated. Watched, but tolerated.
Then in an instant they were all gone. I took my eye away from my camera's viewfinder and looked around. The Robins and the Flicker were gone. The Waxwings were all gone, as were the Finches and Sparrows.
I then looked up in the sky, but saw nothing amiss. I looked around and finally noticed about 75 meters up the path, the shape of a small raptor was sitting in a tree (from the photos likely a female SS Hawk).
In other words, while my presence was tolerated in their midst, the presence of a Hawk some 75 meters distance resulted in a positive threat assessment and all species left the vicinity immediately.
So, from a bird's perspective, where do I stand on the "threat scale"? Even with my big lens? Pretty low down, I expect.
In other words I don't think that we humans "stress" birds much at all. Birds make many decisions every day and many of these decisions are "threat assessments". In the main we aren't seen as serious threats.