I believe they say you should not feed because of the hormones used in raising chickens.
With that said, I boil the shells, put them in the blender with water, and turn them into the finest powder my blender is capable of. I pour the concoction in their outdoor enclosure.
The USDA has prohibited the use of hormones in chickens for 30+ years.
In factory farm situations they are fed an antibiotic (is it tetracycline?) instead.
