quite agree ybs mature too fast , also most reckon the young bird sickness was created by using darkness , if ybs are raced on darkness i believe it burns them up and they fall like flies as yearlings , most continental fanciers specialise is either young birds , sprint , middle or distance racing , very few do all , if they specialise in yb racing they don't keep them after racing , perhaps they already know this burns them out , but the effect of darkness and light ybs on a transporter is this they tend to break up into groups as the darkness and light birds will leave natural birds standing , perhaps this is why some loose birds , but I bet the majority of lost birds are darkness birds .