haven't time to read all that was already posted, but here goes:
Meal worms are easy to raise and already mentioned - high protein, which is the expensive part in most feed formulas.
You can procure pure strains from one of the land grant colleges in Texas (sorry I can't recall which one, but they maintain an extensive library of known algae), but spirulina is a literal super food, high in protein and loaded with all sorts of important enzymes and amino acids.
Blaptica dubia or dubia roaches are a large flightless variety that is a favorite with reptile breeders, again, good insect protein and easy to raise on spoiled produce, they won't climb out of plastic shoe boxes (but still keep a lid on them).
Black service fly larva; there are a ton of different ideas available on-line for setting up screened cages to collect larvae, as well as a bunch of different ways to harvest and contain. More work than the worms or roaches, but it's nutritious, and the birds love the little maggots.
Sprouted seed fodder - various types, but birds seem to prefer sprouted wheat. There are a lot of fairly spendy turn key systems available, but check out Youtube for ideas on all sorts of DIY solutions. Basically your making whole seeds more digestible while also adding carotene, which is the photosynthetic pigment in plants that animals can't synthesize, but that give spring and summer eggs that gorgeous dark orange yolk and what makes Golden Guernsey milk slightly yellow.
I can't speak to the nutritional value, but composting worms, or "Red Wigglers" are very prolific garbage and organic waste eaters that crap out wonderful worm castings to build soil, and oh yeah - chicks dig 'em.

Somewhat related - read a great book that was originally published back during the Depression titled Fresh-Air Poultry Houses that details the particulars of allowing chickens to be housed in open air coops that rely on the birds natural physiology rather than our forced attempts at trying to make them comfortable and "safe". Turns out our great grandparents were a lot smarter than us, and we probably should have spent a little more time listening rather than talking. Anyway here's a link to an edited version (I have nothing at all to do with the author or the book) that I bought since it's long been out of print.
https://www.amazon.com/Fresh-Air-Po...ords=fresh+air+chickens&qid=1584759828&sr=8-1
I also shoot squirrels out of the oak trees above where our birds forage, and the little raptors reveal their inner dinosaur when they hit the ground, ravaging the bushy tailed rats for that free protein. Always a big with our girls.
For anyone who raises fish, they also love fish offal, but be careful not to allow them too much or the oils have the potential to flavor the meat, and I suppose the eggs as well.
Somehow, our ancestors managed, even though there was no Purina store down the street and
Tractor Supply wasn't even thought of.
Good luck everyone. I know a lot of families who elders managed through the last Great Depression thanks to their "backyard chickens", so maybe think about adding some ducks and guineas, along with a pigeon coop, and maybe an aviary of doves or quail. Birds and rabbits may be our new normal, and to be quite honest, that and a little Victory Garden might be just enough to help see us all through.