I am feeding dead house flies to my chickens: I'm catching them in a trap that just uses stinky water with a bit of detergent. I'll go out in the morning and there are thousands in the jar, I remove the lid and pour the stinky water through a strainer into another container. Then, I just toss the dead flies on the ground. They get eaten. (What's scary is how much better the trap works after it goes through a few cycles and the water is not only stinky but also yucky looking.) I went through multiple strain/dump/re-use cycles with the first bait, then I bought a two pack for more bait. When I switched to the new bait, success dropped off for a while, but after I'd caught a few hundred flies it started working really well again. I'm still on the second bait, I have one more to use. The ones I bought smell like rotten eggs, reviews on Amazon say the bait smells like roadkill. Obviously, this is only a backyard solution.
The one I bought at Coastal Farm Supply was similar to this one.
(If anybody know how to turn that into an affiliate link, please do! I do highly recommend this style of fly trap.)