I have a homemade videotape that I bought from a guy who filmed nesting bluebirds. He mounted the nestbox against his window, and changed out a side of the box with one-way glass. One of the many amazing things he filmed was the mother bluebird as she "popped" blowfly larvae. Yes, it was icky. That could that be what you are seeing. They will attach to the nestlings and if there is enough blood loss, they can die.
BlueBars

This is a Quote from your other post

"There are no obvious signs of physical trauma as in a HOSP attack. But their little bodies look rather greenish as if diseased."

"Yes, I did spray my yard with weed killer. And I did scatter grub killer all over my yard."

I would say this is for sure your problem.Healthy Baby Blues are not greenish and don't die all at the same time for no reason.

Matt

Yeah I agree with Matt and was thinking the same thing. If it were blowfly you would find lots and lots of them in the nest. I had a nest at the park with five baby blues that fledged fine and it was full of blowfly larva, with five there is a great chance of survival. When we had three in a nesting though the blowfly larva killed two of them I did a nest change and the remaining one survived. The babies that died from the blowfly did not look greenish at all, more whitish/yellowish from being drained of blood. I would think a greenish color would lean towards something in their system that shouldn't have been there like the pesticides. I must admit I cannot get my dh to not spray our lawn however thankfully he never does it when we have baby birds, only when there are eggs, I am not happy about it either though but he cannot stand dandelions.

Donna

Donna, you should tell your husband about Cornmeal Gluten. It is the by-product of corn syrup production and it is used as a weed preventer and fertilizer. You apply it when the forsythias are coming into bloom and in the fall, too. It is not magic but after several treatments you will have way fewer dandelions and a really nice lawn. I have been using this for several years now, and I can almost manage the few dandelions I still have by hand weeding and a bit of spot spraying. It's better for your family, the animals that live in your yard and the earth.

Just for the common knowledge. Bluebirds forage and eat bugs from grassy areas, they do not generally forage for insects from trees or shrubs. In fact a large number of birds eat the bugs in our lawns and gardens. Be very careful when treating your lawns.

Not to make you feel bad, but just so it is CLEAR....there is a good chance they were poisoned from your own yard. We all know you would never have meant to do this. BUT something to think about...Where are your priorities? If you are insuburbia and the look of the yard is the main concern just dont offer nest boxes, chances are your neighbors yard will kill them if yours doesnt. If you have ample hunting grounds and want to host them you will have to give up the perfect lawn.

If you mow the dandelions before they go to seed you will at least stop their spread. Weeds in my lawn are treated with a lawn mower and maybe an occasional hand pulling.

If you use corn gluten meal, skip the fertilizer. That stuff is 9% nitrogen.

Linda C

Thanks for all the replies folks. The thing about the weed killer and grub killer, I put all that on well before the eggs were even there. So I was hoping there wouldn't even be any grubs for them to feed their chicks. Haven't seen the corn gluten meal around here yet.
As far as keeping the dandilions cut to minimize seeding, I have some neighbors that almost seemed to have planted dandilions on purpose. They have easily 90% dandilion yards. So even if I keep mine cut, I still get their seeds.
Another thing is, I live right next to a few thousand acres of forest, with a mega population of moles that tear up my yard every year.
And the blowflies are REALLY bad this year. We can't stand to be out in the yard for more than a few minutes. I have to run to my truck. So I'm still not sure on the cause of this. But I did clean the box out the other day, and yesterday the BBs already started building another nest. But I'm almost tempted to take the box down for a while to prevent it from happening again.