Stress in chickens

Chickens exist at the lower levels of the food chain - as such, they are constantly on the alert.
They are predisposed to flight if startled.  Anything out-of-the-ordinary causes stress.
Stress causes chemical changes in their guts; these changes lower their pH levels.
The lower pH provides a more suitable environment for 'gram negative' bacteria to grow and multiply.
Because of this, a stressed bird is more prone to disease.
Keep your hens calm and happy!

I actually think it's possible prey animals have less stress under "flight" conditions than predator animals.  Think about it, a prey animal is always on the alert and ready to flee - if this state of being caused stress, they would be stressed-out constantly.  But being prey animals they are adapted to these conditions so they don't tend to experience as much stress as a predator animal under the same conditions, to whom flight is not a common state of affairs.

I believe your both right 

Birds will not have a continues state of stress during 'normal' prey animal conditions, however if they have very extreme weather changes, habitat changes, routine changes and predation you will see stressed birds for a time.  Even so, they are geared to adapt and will come out of it.

There are some situations where stressors can multiply and/or continue and not relent, in this case John is right.  The owner had better step in and manage the situation so the birds can balance out again or their health may be compromised.

As for over protecting/pampering one's birds - I believe this will backfire.  Animals, humans and even plants need a little stress and challenges to be their best.  I feel the key is to keep it all manageable, for us and our birds.

At what point does predation compromise our animals health.... interesting question for sure.

I certainly agree not to pamper the flock.  Birds living in Eden will become complacent, fat and lazy. 
"Hey, Henrietta, check out that cool fox.  Isn't he pretty?" <GULP>
We will still benefit from the reduction of stress in our flocks, and it doesn't take much to stress a simple minded bird.