I plant comfrey because it's a wonderful plant for the birds and for your garden. It concentrates potassium, calcium, iron, magnesium and several vitamins in it's leaves and is therefore usually grown as an additive for your compost pile. You can also line the planting holes of potatoes with 1-2 comfrey leaves to discourage beetles and add potassium to the soil.
Comfrey has been linked to liver toxicity, though, so do not feed large amounts to your birds. I use it as a once in a while treat since they really love it and it's really nutritious with tons of protein.
Look it up on wikipedia or do a google search for comfrey companion planting for more info.
Comfrey is a hardy leaf of a plant that is great in minerals and vitamins for all that eat it including the chickens. When it is fed back to the garden in a composting form it is high in potassium, calcium, iron, magnesium and also nitrogen for giving back to the soil for a healthy and hardy garden.