Of the four original ISA Brown hens I started with, I have two left, who are both just over three years old. One died at a year old, and I put another one down last year due to similar issues as this latest hen. The other 6 hens are all just over a year old, and mix of other breeds. I have heard that ISA Brown hens don't usually live as long as other breeds, and usually succumb to reproductive issues often, and earlier than others. I'm not saying there isn't something wrong with my setup environmentally, but I don't know what it would be. I do occasionally give them meal worms, and I do sometimes let them free range with supervision. I have a lot of hawks and even bald eagles, so I don't let them free range. Water is changed out daily. They have plenty of oyster shell (free choice) and grit available.
Ok, so it sounds like they have very limited access to free range. When was the last time they went out and have you noticed any other issues with her before the 3 days ago, such as weight loss, puffed up feathers, hen off to the side sitting by herself etc?
Has the existing run been wet from rain over the past few weeks, etc?
If you would like to treat her, there are some possibilities of what it could be, such as obstruction (full or partial), sour crop and lost motility, as well as some others.
Treatment will really depend on what level you are comfortable with etc. I should also ask if you have a farm vet that you work with in your area? They can be very helpful for diagnosis and perscriptions that can work extremely well.
If you are treating her, as a minimum of care, I would keep her warm as you are doing, and offer her vitamin/electrolyte and a sugar water mix. You can offer her a liquid paste of her food, in addition to a Tbsp of plain greek yogurt as well. Orego Stim is also great to have on hand. For critical care food, I have had great luck with this product
https://emeraid.com/vet/emeraid-omnivore/
If you go further with treatment, it is likely she may need some of the following until she can be diagnosed:
-Her crop needs to be emptied. I personally prefer the vet do this, as there is a risk of her aspirating the fluid if done incorrectly.
-After emptying the crop, either you or vet will need to observe how it feels then, and in the morning. Please note that sometimes a vet cannot feel an obstruction, even though one is present. I had a 7 week old chick swallow a binder clip, which was later only detected through xray.
She is thriving now, but if left in place, full obstruction would have likely occurred, and possibly zinc poisioning. In your case, an xray may be helpful as well.
In the morning, she should be on a liquid diet (noted above is one that works great). You can then see if motility starts to improve.
-Crop motility - medications from the vet can be used to re-start the system.
-Antibiotics including clavamox pills and SMZ TMP liquid can be used with high success for infections, but will need prescription. I personally have had great success with these and they were precribed by the most advanced medical center in my location. My Brown Chicky cleared up within days of treatment and recovered from something similar to what you describe to live until almost 6, when a bobcat got her.
-Fluid from the crop can be cultured for yeast - Nystatin is often used to reduce if sour crop is detected.
-Ideally, she would get some sub-q fluids to help her feel better, maintain hydration and stabilize her. I prefer the vet do this, but it depends on how skilled you are/level of committment etc.
-They can also provide pain meds, if she is in pain.
Again you have to decide what's best for you and your situation, but don't feel pressure.
