Opting out of industrial agriculture and keeping your own chickens is an environmentally friendly choice already, so a lot of choices you make about your flock, such as which breeds to keep, are neutral from an environmental perspective. However, other choices can help your little farm be as green as possible.
One of the easiest ways to make a chicken coop environmentally friendly is to combine it with a fertilizer, so you can repurpose their waste to replace some of the chemicals used in gardening. Chicken manure is high in nitrogen, so compost it thoroughly with organic material, such as leaves or hay, before spreading it on your active garden. Or, spread fresh manure on your off-season garden. Because of its high nitrogen content, you won’t need to spread it as thickly as you do other manure-based fertilizers. Research how much nitrogen your plants need before you get too heavy-handed; some common garden plants can easily overdose on nitrogen, and all of your plants will perish from receiving too much nitrogen at once.