Grubs, maggots/soldier fly grubs are easy to grow. You just put some old meat out and the flies lay eggs and tada you have grubs. I have seen systems were people mount buckets high on poles, fill them with meat, and the maggots drop out of the holes on the bottom right down to the waiting birds below.
Once you get a system going, i'd say earthworms would be a good one to grow. They propagate pretty fast, need food almost everyone has, and can be grown in a basement or shed fairly easily. To grow enough for 60 birds you would need a pretty big system though.
Leeches would not be a realistic food source unless you have a good source of fresh blood. To grow enough to feed any bird, you would need to also grow a lot of fish to feed them. Not really, but they are hard to feed. I keep a few pet leeches (i know, i'm crazy) and they are easy to feed when there are only a few, but enough to feed chickens? no way.
Crickets could be a good food source, but are hard for some reasons. First, they need a large system, and fairly special setups. To breed they need to be kept warm and babies must also be kept warm until they are large enough to feed. This equals expensive setup and lots of electricity. The poop a lot, so you are constantly cleaning (especially if they are in the house, crickets stink), and they don't live that long so half the babies will be replacing the parents as breeders. IMO they are a lot of work for chickens.
Grasshoppers are REALLY hard (unless you pull the legs off). They jump really well, so everytime you open the cages, you lose half of them. They don't breed easily and are slow growers. They are more pick on food than crickets as well. If you must do cricket or grasshoppers, do crickets.
Also not feasible. I breed snakes, but they are a challenge and it would be soooooooo expensive to breed snakes as food. Snakes require tons of space and each setup require a heat source, water source and daily care. This would be even more expensive than crickets. They also need mice, so you would essentially have to have a mouse breeding setup to feed the snakes as well as the snakes.
Ok, here is what i do. I breed mealworms (easy to breed, totally reccommend) to feed the chickens, ducks, and turkeys mostly, but also feed a small percentage to my many Tarantulas, lizards, and turtles. I breed earthworms for the chickens, ducks, turkeys, garden, Tarantulas, lizards, and turtles. I breed crickets to feed some of the lizards and Tarantulas. I breed mice, rats, and gerbils for the snakes and the chickens (they like them). That is mostly what i do.
I also breed Tilapia and Trout for my family to eat, and the chickens LOVE the guts, heads, leftovers. They also get dead minnows (another thing i breed for some of my pet fish and my aquatic turtles.
Sorry for the long post...hope this helps a bit!
