My girls are 17 weeks. We're just about to finish a bag of pullet feed. Is it time to switch to layer feed (noticing combs getting redder) or wait until they are actually laying? Same with oyster shell? I don't want to start a 50lb bag of pullet feed if they are close to ready for layer feed. Obviously I'm a newbie They are black australorps, buff orpington and golden sex-link. Thanks!!!