Before you start giving scraps make sure that you are giving them layers meal or pellets as a first choice and available all the time..and then some scraps as treat..little every now and then..
I never give any treats for first few hours of day as then they will fill themselves with layers pellets first and they will get the basic nutrients in them..
As for scraps..oh..easier to say what not to give..
NO... for any foods containing poulry and bird game...Salty foods...anything that is off or mouldy
Be steady with anything containing fish..as it may make their eggs taste fishy.
Lettuce only in small amounts as it will give your girls runny bum

Other than that..just have fun with trial what they actually like. Each lot of chickens I've had,,their taste buds been different..even between individual chickens..some prefer greens, some bread, pasta or potatoes..some berries..
but generally chickens are not fussy..once they start laying anything goes and they will just eat and eat

..get your sprinting shoes ready as you will find yourself running into kitchen to find something to give for them while they are screaming their heads off to rush you
