That is great news!Dr Louise called and tells me Katie's kidneys are much improved! Her reading was 522 and 480 is the upper normal value for chickens. So that's brilliant!
That suggests her kidney injury was acute (regardless of what caused it, zinc toxicity being the most likely candidate).
The vet then suggested giving Katie her own breakfasts to address the weight loss, which is what I was already thinking. So each night I'll take her from the roost and put her in the big coop, which I'll stock with a feeder full of seventeen, which is a crumble with corn for adult hens. And some water. She can come out and join her friends after breakfast.
In a fortnight, she has another kidney and weight check. Hopefully we'll discover her kidneys are within normal range and that she's gained a little weight.
