Thanks for the tag
@Nabiki , without clearer pics, it's really hard to tell what's going on there....I see lots of poop on the ground but can't tell anything about it.
I'm going out on a limb here but it might be Ulcerative enteritus. Although, Coturnix aren't as prone to getting it, it can be spread by wild birds visiting the pen. Also, as stated, "lots of poop on the ground", that's how it spreads, through dropping in the feed and waterers.
Without a necropsy being perform it's hard to say just by looking at pictures.
If a necropsy is performed, you would see lesions, inflamed intestines, if it were UE.
Either way, I'd start the entire flock on Duramycin, erythromycin or tetracycline, immediately. UE is very contagious and can spread through a flock like wildfire.