I was reviewing this thread because... well, the kids start fasting tonight, and it's been 30 years since the only time I've done it. One thing I noticed coming up in a number of comments is chicken's pain experience, and while not a scientific study or anything like that, I wanted to point out a few things that might be reassuring.
For those who have processed - do you recall any reactions in the chickens that you would interpret as pain? I have not; I once had a bird that seemed determined not to 'cross over' no matter how long/much he bled - but even he didn't give any indications of experiencing pain. Now, the plucking of the area may well be different, and that I would kind of expect, given how they react to other birds having a pull at them.
Another reason I believe there are limited pain receptors is recent. We've developed a fox problem, and my flock is VERY free range. While I've lost five in a total of four attacks, I've also had a good number of ladies who were barely survivors. Some just lost large areas of feathers, but seven of them also had slashes from teeth; one had the peritoneal cavity broached. I have gotten more repair experience this spring than I care to have had. In all cases, once immobilized, the hens did not struggle nor react vocally while they were being cleaned/glued. The only reaction was to the blu-kote spray after the wounds were closed. Note that I used saline as a wash, no alcohol product was applied until the skin was closed; I'm confident that alcohol based cleanser would have elicited a similar response during 'surgery'. (and yes, all still survive, the broached lady was missing a quarter-sized piece of skin, which is now a dime-sized scab after two weeks, and never stopped laying)
Just my thoughts/experiences.