Okay - here is the story........My chicks were going to hatch 3 days apart under an old silkie hen. We have hedges around our coops and lots of wild birds that try to sneak in and get the chicken food (though we've got a set up to make that not as available as it used to be). I had an old silkie hen sitting on 4 eggs in the nesting box. The first chick hatched on Monday, second on Tuesday and then the 3rd and 4th eggs hatched Wednesday. Three sizzles and one silkie. I was thrilled. Late night on Wednesday I noticed on my chicken cam, in the nesting box, that the hens seemed irritated that night like little gnats were flying in there or something. Made a note to check it out in the AM.
First thing in the Am I was out there. To my horror the last 3 hatched chicks were dead, cold to the touch and the 1st hatchling, the oldest, was fading. I held it to remove the dead chicks. The hen freaked and left the nest and wouldn't return. So, I brought the baby in and fortunately had a set up conveniently stored in a cabinet for raising newly hatched chicks. I set it up, raced to the feed store and got some electrolytes for the chick. I began giving it tiny bits of liquid not knowing what else to do.
At one point, I saw a tiny bug on the chick - a mite. So, I went on line and posted "would mites hurt chicks". Immediately my research showed that mites will kill chicks faster than anything. The one chick was surviving thus far because it was the first hatched and had been the strongest. It was recommended on line to take a scant amount of sevin dust on a q-tip and dab under each wing and at the vent. I did so and kept giving the chicks tiny bits of fluid throughout the day. When the chick was resting, I went outside and tore my coop and nesting box apart. I put sevin dust everywhere. I'm an artist and have this big fluffy paint brush in my collection of brushes. It looks like a cotton ball with bristles and is very soft. So I put some sevin dust in a paper cup, and on this dry brush, swirled the sevin dust into the bristles and then swabbed under each adult chicken's and rooster's wing and vent areas. I did this every week for 3 weeks (as it was suggested in the post I was following to kill eggs and prevent future mite problems).
For the inside chick, who by the end of the day was starting to raise it's head, I did the tiny dabbing of sevin dust 3 days in a row - just a scant amount of sevin dust on the wee tip of the Q-tip. By dinner time that very day the chick was sitting up. By bed time the chick was pecking at food on the paper towel. The chick was going to pull through.
Had it not been for seeing the one little mite (just a fluke - or luck) on the surviving chick I would have never known what killed the other three hatchlings. I thank the Chicken Forum for the wealth of information shared here. I have one fabulous 6 month old pullet named Miracle as a testament to it.
Now, because we do have a lot of wild birds in our yard, I completely change out the nesting boxes weekly, dust with Diatomaceous earth and regularly, but not frequently, will sprinkle some sevin dust around. No more mites and we have two clutches of chicks thriving out there right now. Tough lesson. And, my coop was clean - I keep it clean but especially clean because I was expecting the babies to hatch. It all happened so quickly. Still blows my mind. I always fluff a little DE into the shavings - always.