Adding to what nelson96 has posted in response to vardaman:
Chickens are omnivores. So are turkeys. The seed heads of grasses/grains are concentrated sources of nutrients. Corn, like other seed heads, is highly concentrated energy. Although the low-carb/paleo paradigm does not recommend that HUMANS make these starchy, calorie-dense grains a dietary staple, poultry can and do utilize these efficiently. They are NOT ruminants. Chickens do not eat grass and make eggs. They eat grass/vegtables, seed heads/vegetable seeds, worms/bugs of ALL kinds, fungal hyphae, shrub buds, sprouted/immature seedlings, roots, soil, and WHATEVER ELSE they can get their beaks on. I know because I raise chickens and have seen them consume all these things, including mice and their own eggs (if the egg gets cracked there's an instinctual feeding frenzy). If confined in a chicken tractor, coop, fence, or cafo, a chicken will need good concentrated, balanced nutrition to produce eggs....since they are unable to harvest all their required calories/nutrients from Nature's banquet. Chickens only produce when all their nutritional needs have been met. Feeding confined chickens corn/grain is a convenient and relatively cost-efficient way to provide them with those calories. Additionally, protein is absolutely critical. The most common way to provide this is with some kind of soybean, with roasted being the preferred form, or OTHER pea or bean (yellow pigeon pea, fava bean, etc.) There are other options for protein, like meat scaps or dairy products (if you're feeding a backyard flock). Then there are mineral considerations, with calcium being of primary importance.
The BEST way to control your egg supply is to keep your own chickens and feed them according to their nutritional needs, your budget, and your values. The next best way is to find a local farmer or backyard flockster and patronize them, since they will be able to tell you exactly what they're feeding.
As for antibiotics, I don't feed preventative doses of them to my layers or broilers. Only if I observed a flock-wide outbreak would I treat for something like coccidiosis. I have losses. Some chickens die if they succumb to a disease that others are able to shake off...much like people. The pastured model can be tough on birds. My broilers are huddled in their pen because we're having a cold snap. If they were in a climate-controlled cafo, they wouldn't be under any temperature stress.