A reptile forum isn't likely the best source for bird breeding information.. BUT if their eggs are anything like reptile eggs, fertile eggs will likely see veins if you shine a flashlight through them, and infertile eggs won't.

If all you see is the yolk, I'd guess that they are infertile eggs. From what I remember when my mom had birds when I was a kid, it's certainly not uncommon for birds to lay infertile eggs, especially for a first clutch.