Corsish cross broilers are going to be more tender than any other breed. But I have found that if I slow cook (either in a crockpot or on low on the stove) with enough liquid to mostly cover the bird (keep turning any sections sticking out down into the pot every couple hours) they will get so tender they literally fall off the bone. It works for freshly killed birds as well as frozen. But...it takes HOURS to cook thoroughly to be tender. Occasionally there will still be a chewy strand across the outside of a thin section of breast depending on the breed & age of the bird.