So I just found this...
"Sudden death syndrome is a condition associated mainly with commercial broiler systems, where apparently healthy birds suddenly die for no apparent cause. It tends to occur from 72 hours post-hatching up to 12 weeks of age, with the greatest losses occurring between 2 and 3 weeks.
The condition is also referred to as flip-over, acute death syndrome (ADS), dead in good condition, heart attack or lung oedema. It is associated with fast-growing broiler birds and is therefore unlikely to be a problem in organic table birds.
There are no clinical signs or unusual behaviour until less than a minute before death. Then there may be a sudden squawking, loss of balance, convulsions and frantic wing flapping. Birds tend to die on their backs (hence the term "flip-over"), with one or both legs raised.
At post-mortem birds show an enlarged, pale liver and kidneys may be pale and lungs congested and edamatous, although the latter may be a postmortem artifact.
As part of Defra-funded project AW0221 “Extensive table bird production - health and parasite status monitoring”, ADAS (2002) showed that in medium to fast growing breed-types, high specification rations tended to increase the incidences of ‘flip overs’ and leg culls."
...on organicvet.co.uk. I linked into it from this site. This is exactly what happened to my chick, except that I'm pretty sure it was an austrolorp. I thought they were more medium sized egg layer types.
I've also been reading about something called 'Early Poult Flip-over' over on the Diseases and Emergencies forum...which is probably where I should have posted this question, I guess.
I don't know what to think. I just hope the rest of them stay healthy.