Thanks, everyone

I started with a silver laced wyandotte (she died), an Americauna (she died), and a frizzle-something bantam (who, we later found, was a roo). So we've currently got a barred rock (my favorite, so far - she's so sweet, docile, and just plain pretty), another Americauna, and I'm not sure what the third is. My eldest tells me that the lady we got her from said she's a white leghorn, but, other than being white, she doesn't look much like a leghorn (from the pictures I've seen).
That would've been good info to put in my original post, thanks for asking.
To clarify, the two that died, my hypothesis is heat. Coop is in direct sun (though they have plenty of access to shade in the run and while free-ranging) and both were found in the nesting boxes. I have to assume it's because they got overheated in there and were too distressed to get out in time. I've since provided a shaded nesting box, drilled 8, 1.5" holes in one side of the coop, and have ordered a small fan to attach to the other side. Also added four additional windows to the front of the coop. Hopefully that creates enough circulation that it doesn't feel like an oven in there.
Until that fan comes in, I'm placing ice packs on the sides of the nesting box that's in the coop, in case they do decide to use it.
However, if anyone has other suggestions, I'd love to hear them!