I think there can be issues on both sides when it comes to fertility though its usually considered the male problem is the hen is laying nice eggs.That's a good point bringing up the shell quality of younger birds. On the same token the harder shells might make the offspring work too hard to crack out of the shell. The one year mark is considered "adult" for chickens except that with laying hens many show reproductive problems between one and two years old. Having good records to show young hen and older hen progeny progress must be quite a task. So many things to consider when breeding which is why I don't do it. Can't anyway because of my zoning but I'm still interested in seeing what owners are trying in their projects to get rare breeds improved. And what causes fertility issues other than a possible chicken disease/malady? Is it the problem with the roo or the hen?