No matter the specie, if it is barren or sterile, it eradicates its own problem. Scenario: Keep going the way you are with the occasional barren / sterile bird. Eventually score 1st Open National Gold Cup or Medal. Fill your loft with progeny from the winning bird and line breed to him. Suddenly discover he was a carrier of the problem and everything you have in the loft is producing barren / sterile birds. I accept fully that very few will ever own a bird good enough to receive this sort of treatment and that very few are skilled enough to line breed to it, however the situation could arise. That is the reason for noting the breeding of any bird which is barren or sterile. If a recurring pattern is found, then that line should be no longer bred from. I am not talking about remote incidents or knee-jerk reactions, I am talking about careful considered reaction to a problem within a line once sufficient evidence has been collected. I am also very aware that very few line breed nowadays, but in the past most successful fanciers had their own family of birds cultivated over a lengthy period of time.