Hard Boiled and Missing Eggs?

Well today is about day 15 of incubation and no sign of babies. I am REALLY getting worried. I have been worried because of this extreme heat and the fact that the female has been sitting on the nest more then she did last time. The facts I have been told state that if it is hot they don't sit on them much. My mom thinks because of the heat they may be hard boiled by now. I thought she was just making a joke but now that I have been reading on http://sialis.org/index.html, I am thinking that may be true. The website says that extreme heat can be fatal to the eggs or babies. I wish I had read this before today as I could have put up a heat shield of some sort. This morning I saw the female fly out so I went to check the nest. All 4 eggs were there but one looked a little funny. It has gotten thin looking at one end and looked like a faint white ring at the top half of it. I thought it might be a sign of it about to hatch. I noticed the male and female hanging out on the fence all morning long. I tried to go out again but the male attacked me each time I tried. lol. So I stopped for a while then around 12:30 I tried again and still 4 eggs. No change. Then I noticed them still hanging out on the fences this afternoon. I checked the nest yet again. 3 EGGS????? An egg was missing!!!!!!!!!!!!! I am sooooooo confused. I have never seen a snake in the yard. We don't have cats or squirrels. I watched a lot this morning and the only thing I saw was our resident Mockingbird and the male Bluebird was not happy about him being around so he was pecking at him. The mockingbirds have a nest in the neighbors tree in the front yard. I know they are aggressive and territorial but would they take an egg from the nest? I am currently trying to get some advice from a forum http://nature.gardenweb.com/forums/bluebird/. I am really worried and disappointed but am holding out hope for the remaining 3 eggs.

Male and Female Sitting on Fence

12:30 4 eggs

4:30 3 EGGS!

Nest Boxes and Redbud Tree