I can only say this has been a weird season for me as well. In my years of hosting Bluebirds, I have never had an infertile egg until this season. My Blues first nesting attempt was five infertile eggs. Like you, I candled them once they were well past any chance of hatching. Then nesting number two, she laid six eggs. All hatched and fledged. Nesting number three she laid five eggs. Only two hatched. After the two were five days old, I candled the remaining eggs and they were empty.

Thanks for your reply, Kendra. I've been around this forum for 10 or so years and have had only one attempt in a nestbox on my property in all those years. Then all of the sudden this pair shows up much to my delight and produced 9 infertile eggs in two clutches. And that seems very strange that you have experienced this problem this year yourself.
I was just informed by my husband that the blues are attempting a third nesting; they are currently building another nest. Fingers crossed.... I will candle these eggs much sooner this time.

I sure hope clutch number three works out for you. After all these years and finally getting a nesting pair, this must be so frustrating for you. Last year my Blues fledged 15 in three nesting attempts. This year it will only be 7.

Hello, I've come on to try to figure out this problem for our bluebirds, as well. This is the second straight year of nothing but unhatched clutches for our bluebirds. We finally got our first bluebirds nesting last spring (2013) and they built 3 or 4 clutches, none of which hatched. They are now working on their 2nd clutch this year, and no luck. A total of about 20 to 25 eggs, none hatched. What to do?