6-Day old chick extremely sluggish.

[TLK777]

I hatched out a dozen baby chicks about a week ago, and one in particular has been sluggish since he came out of the shell. He moves around on occasion, but for the most part, he spends a lot of his time sleeping face down. Obviously since he's made it this far he has to be eating and drinking, but his behavior is still questionable. For a while it seemed as if he just had to get used to being outside of the egg or even his size (he's pretty big for being a newborn...although it seems like the other chicks are starting to outgrow him pretty quickly) but he's been particularly sluggish in the past 48 hours. None of the the other chicks are displaying any sluggishness and are eating and drinking well. I have since quarantined him as a precautionary measure in the event that he is sick.

There are no visual signs of deformity and he chirps like the others when he's handled or disturbed...other wise he's usually passed out. I wasn't there to witness his hatch, but I guess it took him a bit longer to hatch than the others. Since he's made it this far he has to have had some form of food and water intake, but I feed him some egg yolk / milk / sugar mixture last night, and he has a plentiful supply of food and water in his confined pen.

Any idea what this could be?