I was a little disingenuous when I said that we were going to raise old-fashioned chickens because back in the day, I am not sure people intentionally used the compost pile approach. It was more that chickens roamed around, fed themselves, and happened to find compost piles. The compost and chicken system that Karl Hammer at Vermont Composting invented and that I have now implemented at a backyard scale is not historical; it's a super-cool innovation.
In this new system of feeding chickens on kitchen scraps you put in way more scrap, leaf, and green material than the chicks can eat - 2 pounds per chicken per day (we use our own scraps as well as those from three neighbors). This runs squarely counter to conventional wisdom, which admonishes to give the chickens only as many scraps as they can eat or the left-overs will turn into a slimy pest-attracting mess. So what's the deal? Why don't we get a big, slimy mess?
The secret is: what we're doing is not just feeding chickens; we are creating a big compost pile. Anyone who composts in their backyard knows that compost piles - without chickens - don't smell or get slimy as long as they are turned and have a good ratio of greens to browns. For starters then, let's think of our project purely as making a compost pile. We pile garden waste and food scraps on top, turn it every now and again, and when necessary, add some leaves or other carbonaceous materials - compost business as usual. The chickens are just helping us. They eat some of the scraps and what they eat comes out the other end as fantastic manure, plus they constantly aerate the pile with their scratching. It's a compost system deluxe. Here is a good article on it. Oh, and by the way, the chickens lay eggs too!
We throw all kinds of things on the pile that are not supposed to be fed to chickens according to the literature, including moldy food, citrus rinds, meat and onions. The chickens are not dumb; they don't eat the bad stuff, they focus on the plentitude of delicious treats. The rest they leave to decompose in the compost pile. (However, when our cat killed a baby rabbit, I did bury the corpse deep in the pile, since I did not want the chickens to directly eat it, and preferred it to provide nourishment for pill bugs. I don't actually know what happened to the rabbit, I never found a trace of it ever again. Was it decomposed in a week in the pile? I don't know.)
One piece of work I do engage in is to turn the compost piles. Sean Dembrowsky from Edible Acres, whose terrific chicken series I watched on YouTube for hours, likes to roll his summer compost piles from where they start with fresh food scraps, to the almost finished compost. I've adapted this into a three-pile system. The first pile is where the fresh scraps go, the second is an intermediate pile, and the third is where the compost hangs out the longest. Every week or whenever I feel like it, I will take the material from the intermediate pile and put it on the third, finishing pile, and then move the fresh scrap pile into the number two spot, leaving an empty area for the new scraps. This exposes lots of worms and bugs for the chickens (they are all over it!), and designates one pile to be the one that I can remove for compost use in the garden. I believe this system will change somewhat during the winter, when the piles will include a few cubic yards of leaves, but that remains to be explored. Other folks just continually throw new material on the pile, and move the newer stuff aside to get the older, good garden stuff on the bottom on an as needed basis, so that works too.
There are so many worms and bugs in this turned-over compost that I wonder whether the chickens' work is actually creating a better environment for growing lots of wriggly critters - a symbiotic relationship where the chickens provide worms a better place to thrive, and worms become chicken food. That would be typical of Nature's way of doing things: a positive feedback loop. However, as I didn't find any research on this, it will have to remain my personal little theory.
There, and that's how you feed your chickens for free on compost!