In college, I helped a professor who studied chickens and their poop. Several hours a week I wore a mask while taking ammonia readings in the chicken house. When the ammonia is too high, it causes blindness in the birds. Blind birds don't eat. From what I remember, my professor was trying to figure out a new diet for the chickens to lower the ammonia in their poop.
Those HUGE conventional chicken houses will really make one think about the health of the bird, specifically it's meat & eggs.
Here's a link that supports what I've suspected: free range chicken eggs are more nutritious than "conventional" eggs. Healthier birds produce healthier products. The study, however, wasn't that extensive.
For a while, I've wavered at whether or not to splurge and buy free range eggs. It seems like free range eggs have harder shells and the yolk is darker yellow/orange. They're a bit more expensive, but $2-3 for 60+ grams of protein isn't that expensive.
HT: Starry Sky Ranch