Compared to the birds I've seen maybe a 2. Don't delay treatment and treat her right now. By morning with no treatment it could be too late. Maggots are horrible and do so much damage so quickly. Maggots can eat threw the organs in 6 hours.
Clean and cut the feathers around her soak her in epsom salt and pick out the maggots. A rehabber I know uses capstar and apparently the maggots just fall off! I've never used it before but it's a lifesaver for the animals she works with.
Not sure if you can use it for chickens but I know a...
Maggots attack quick!.if you are in a place with alot of flies and you get a wound you could have thousands of maggots the next day. I've had rescue calls about ducks who have lost eyes and the next day the whole face is eaten. Maggots are nasty animals and if you spot ANY you need to act fast...