Unfertile Eggs

I have a question.
It is now the 4th or 5th breeding cycle since our Emily is laying eggs which are not fertile any more. As you know Emily and Pete are our balcony couple who is living with us since March 2008. I cannot say how old they both are - they were adults when they decided to live on our balcony. Emily has laid more than 50 eggs in that time until today and she and Pete have raised 5 babies, another 2 last year died at very young age. The last baby they had was this year in summer time, wonderful Charly. The other egg did not hatch and was the first to be unfertile. So Charly was the last baby to come. Since then no fertile eggs any more.

Could it be age now or anything else? Pete and Emily seem both very healthy. Emily's eggs look perfect but still they are not fertile any more. Pete is behaving like any cock, courting any hen who appears on the balcony and when Emily is not sitting on her plaster eggs he is mating with Emily and cuddling with her as usual. So I really do not know what to think...

Petra...my guess is that they are carriers of salmonella or E-coli, either of which can cause the babies to die in the shell or die young. As carriers...mom and dad may act just fine. Remember though, birds do pretend to feel okay, even when they aren't as a natural defence against predators.
If they birds were in my care, I would treat them both with baytril or a baytril generic, for 14 days.
I can't think of a natural remedy that will cure them.

Thank you Charis.
I have checked the eggs carefully each time when I replaced them with the plaster eggs - there were no veins to be seen in any of them. So this means to me that there is no fertilization taking place at all!!!!

Salmonell or E-coli could be making the birds infertile.