Howdy
ezicash
Let the cat have the coop and the chickens the warm house
Seriously now, I am not actually sure of a solution but I can understand why your chickens are not using the nest box.
I have a few suggestions which may or may not be of assistance and some are obvious:
1. Keep the cat inside.
2. Secure your coop/run so that the cat can not get into them.
3. What do you think is appealing to the cat in the nest boxes? Could it be the bedding or the confined space? Do you have somewhere inside for her that is confined .. maybe a little ‘coop’ of her own with a nice soft bed?
Is this something she has only recently started doing? I note that she is 7 years old and while I definitely do not want to be a ‘downer’, does she seem as happy and healthy as she always is? Has she had a vet check recently? Sometimes when animals are sick, they find somewhere ‘out of the way’ to hide or mope etc.
Has anything changed inside the house recently? She may be feeling insecure or threatened?
How long have you had the chickens? Is she feeling a little ‘put out’ at their arrival and possible jealous of the attention they are getting?
I do not know if 7 year old cats still have a desire to have kittens, but has she been fixed up so that she cannot have kittens? She may be ‘nesting’?