Mealworms are easy to grow in small containers indoors, or Black Soldier fly outside in summer. Grow duckweed in any container that will hold water. Temperature sensitive, but it will go Winter dormant, encapsulate and sink to the bottom, start growing again as weather warms. These are all good protein sources and insects have the fats poultry need. You can also grow winter squash for them. The seeds are high in protein and act as a vermifuge to boot. The flesh of the squash is a source of vitamins and carbs. My Grandmother always fed corn in winter to “help keep them warm” but not much in summer as it made them too fat, and an overly fat bird is not a good layer. Oats have so much fiber, if they eat too many they don’t eat enough of the concentrated proteins they need and their feathers will fall out. Sunflower is easy to grow, and you can throw the entire heads in the chix pen, they will pick it out. But I’m talking about the larger breeds. Seed might be too large for smaller birds. Same with milo or sorghum seed heads, they will pick it out. You could rotate runs and plant in a run they were not using and turn them in at harvest time. Oregano is a good plant to grow for them to have partial access to so they don’t kill the entire plant. Such as planting outside their run so it grows into the run and they eat whatever they can reach. Keeps them healthy, viral free and kills parasites.
When they are moulting they need extra protein and calcium to regrow healthy feathers and restart laying. Dark green leafy vegetables have a lot of calcium. Dry their eggshells, crush and feed back to them. Also oyster shell or any crushed seashells If they don’t get enough calcium otherwise. Or mix a little calcium carbonate (lime) into a wet food mix, such as table scraps.
Biggest problem with poultry is giving them enough protein, predators also a problem, especially if you free range. Weasels, skunks, opossums, raccoons, foxes, Bobcats, and coyotes plus the occasional domestic or feral dog will kill poultry. Domestic house cats will sometimes kill baby chicks. So will rats.
During the last depression people would collect roadkill and feed to their chickens. Might have to cook it first, but could do that in a pot over a fire outside. You can also cook and feed the offal from any game you or neighbors hunt and dress out.