Meat chicken found dead

I ordered 50 Freedom Ranger chickens.  52 showed up in the box at the post office.  They are now about 2.5 weeks old.  Been checking in on them at least twice a day and they've got plenty of fresh water and food.  One was flat on his back dead when I checked on them after finishing my yard work for the day.  When I'd started the yard work about 4-5 hours earlier all of them seemed to be fine and alive.  It's possible I missed him as he was off a bit to one side.  He was pretty stiff by the time I found him.  However he was definitely still alive this morning when I fed them and changed their water.

What should I be on the watch for to make sure the others don't also die on me.  There were no visible signs of trauma.  

I've gone out to my coop and found a chicken keeled over, same with my friends who raise chickens. It just seems to happen and isn't exactly uncommon. I don't think there's much one can do besides provide food and water. It seems that occasionally there's gonna be a bird that just doesn't make it. If you start finding one or two a week dead, then that to me clearly indicates there's something else causing premature mortality.

Figured it probably just a random death.  So far the rest of them look fine.  Like I said, I don't want to just assume it's a random death, so knowing what to look for in case that was just the first fatality from some illness is important.

I raised 300 cornish cross last summer. I lost a bunch to coccidiosis. I would find them lying on their back stiff. A few I found just before dying, puking up liquid. This was mostly between 4 days and 2 weeks of age.

This could be what happened to yours, mine seemed to develop immunity to coccidiosis around 3 weeks.

I also had a few that were simply unhealthy, smaller than the rest and eventually died.

Most of the meat birds have no immunity because of the breeding done to get them to grow fast, they also will have weak bone structure (Cornish cross for example is ready for harvest in 6 -7 weeks from hatching out).

Coccidiosis is one of the major killers of meat chickens, they can develop immunity but as Leora mentions it takes a few weeks for that to kick in.

Redhawk

All still alive this morning.  Regular feeding frenzy when I gave them the feed.  

Tomorrow they're 3 weeks old.  Hopefully they have that immunity by now.