in my opinion, theres no set average, i think there are many factors that can effect this average, firstly only breed off healthy contented birds, then only keep yb,s that are robust and well reared, from the very offsett, i think overcrowding is one of the worst scenarios, to my mind this only produces stress which in turn can cause health probs, i also think that all young birds "lost" are not all bad birds, there are many many hazzards nowadays, i still think the old adage of never "putting all your eggs in one basket" still goes,, looking at yb racing nowadays i still think its wise to stop some after being well trained and raced 100 miles, in short some good yb,s never get the chance to become good old birds, so the average can become somewhat higher , with good, carefull management,,,only my opinion lol