A couple of issues here.
Treats generally are best rationed to around 5 % of the chx daily bulk requirements. Not all treats are equal but generally they tend to be high in energy density (ie a lot of calories for little bulk). Chx are grazing foraging animals and NEED a staple diet of low energy density food for two reasons.
- 1 Chx regulate their energy input so they eat enough for their needs without over eating but bulk plays a part in this regulation so too much high energy density food can cause over eating and obesity
- 2 They need to stay active both for physical and mental well being so providing the energy input they need without the bulk feed and foraging leads to inactivity and boredom which can promote bad habits like feather pecking. This isn't that disimilar to high energy drinks and foods causing hyperactivity in children
Also too many treats can upset the balance of nutrients formulated in layers feed for optimum egg laying performance. However to be fair they would have to eat a significant imbalance before it was a real issue for pet poultry but commercial layers are more finely tuned to produce optimum feed conversion.
For the above reasons you should not overdo treats and always feed in the evening to top up only. Vegatables are much less of a problem than protein or refined foods like spaghetti
The second issue is meat. The issue here is more to do with the sensitivity to argrocultural food production and problems such as BSE. Commercial meat or egg producers are banned by law from feeding ANY food to an animal which might enter the human food chain with food or leftovers which has been prepared for human consumption in a kitchen. From the regulations I've read there is some ambiguity how this would apply to hobbyists meat or egg production. If you sell your eggs I suspect you would be breaking the law to feed leftovers and in particular meat. If your eggs or chicken meat is for your own consumption I suspect pragmatically the rules aren't enforced though again technically I'm not sure if you might still be breaking the law feeding kitchen scraps. Meat is particularly frowned upon especially feeding meat products to herbivores, though as has been observed chx are omnivorous.
I've heard conflicting advice on this issue and how it applies to none household pets ie pets which are classified as farm species animals (ie chickens or pet pigs for example). You can feed meat happily to your pet dog or cat but I'm not sure the same is true of your pet chicken, as a chicken is classified as a farm species. I've heard conficting opinions even from vets who I would have expected should know the definative answers.
Personally I only would give meat very infrequently or specifically if I wanted to increase protein as a convalescent treatment.
These are my understanding of the legal issues and I'd welcome any input from anyone who produces food for the public and has a better informed opinion.
Best wishes
HF