Hello Andy.
My birds are out free-rangeing around the yards and paddocks on the farm here.
They will do great in spring and summer, as fresh grass tips, emerging weeds and bugs and grubs will make up a good part of the diet and protein requirements all they need besides is a mixed grain diet. This grain will deliver around 12% protein the rest is more than added from range.
A lot depends on the breed and if they are in lay or not. In lay a bird that produces 6 eggs a week should need about 16% protein plus calcium in the form of limestone grit/oystershell and of course fresh clean water add lib.
In Autumn and winter when the light levels drop and the free feed starts to go you need to up the protein levels with greens or animal by products plus cod-liver oil to supplement sunshine and vitamin A-D and feed add-lib, removing at night to prevent attracting rats. When I say animal by products I dint mean ground up dead animals that caused BSE you could used milk in moderation or anything that has a high protein level IF you want to feed animal protein, as long as the birds get some even derived from vegetables they will be fine.
I have a bit about feeding through winter on my web page but you may need to search abit.
