Sudden lameness in King Quail?

I have a hen who is approximately 3 years old. Every once in a while I would notice her gimping around the pen but would always put it down to her injuring her foot. I could never find any sort of lesion, cut, or otherwise obvious injury and the situation would resolve itself after a few days.

Just a few days ago I noticed her doing the same thing but hardly moving from one spot at all. She was putting all of her weight on her chest/belly. Concerned that she might not be able to reach the food & water, I placed her in a small plastic terrarium with small dishes of food & water right next to her. I've had her in the terrarium since Friday but she hasn't seemed to improve.

She has a good appetite, normal thirst, and poops look normal (a little on the white side, but plenty of brown ones mixed in). She can stand upright for short periods and immediately hunkers back down to take weight from her feet. I have no idea what's wrong with her. She was perfectly fine last week. My quail pens have newspaper and fine cedar shavings as litter, a small wooden log to stand on, and their feed in a small dish. They are fed 20% gamebird crumble mixed with budgie seed, grit, and dried mealworms every day.

3 years old is getting a bit old for some quail. They can start to develop arthritis at this age. Take a good look at the feet and joints. Are they getting gnarly at all? Warm? My quail seem to start in with limping and gnarly arthritis legs and hips at 3 or 4 years of age. If you think this is the case, you might try a 15 mins a day infra red heat lamp on her. I do this for my arthritis patient quail and it really helps them to loosen up and walk better. Also, apple cider vinegar helps to reduce inflammation. 3/4 teaspoon in a plastic only quart of water. Change and make new daily. Offer this up for one week straight and then a couple times a week after this. I have noticed my birds improved after doing this.

Also, make sure you newspaper is completely covered and deep with the bedding so they are not slipping on the paper. Newspaper is slippery and can cause sprains and strains, especially if they are jumping down off the log. This could also be the problem if it is not arthritis.

Keep the food and water near her at all times. Keep the bedding deep and try the lamp and ACV. Hopefully one or more of these ideas will get your little quail feeling better soon!

Thanks TC! I've got a low-watt ceramic heat bulb that I'll try on her this evening, and ACV in the cabinet.

It's odd though--I have 4 quail who are all the same age-- ~3-4 years--and they're all pretty sprightly. I'm not 100% sure how old this hen is, only that I bought her and her sister from a breeder back in 2011 and I assumed they were young adults. They could be older!

What type of quail are they?

I ask because different countries seem to call either coturnix or chinese quail king quail.

She's a King Quail, coturnix chinesis: http://www.aviceda.org/abid/images/data/1097998113.jpg