There is a store nearby who will give me beef bones for free. I almost always have broth on my stove with the bones, water and ACV (ACV helps to draw the nutrients out of the bone). I boil it, then let it simmer for 24-48 hours. I then let it cool, and it is becomes gelantinous. The birds enjoy it, along with any scraps that have boiled off the bones. If the bones are small enough and get soft during boiling, the gobble those up too. I figure there is a lot of gelatin and calcium in there, maybe a little protein leftover too. I have also added grains and let it soak overnight for added fun for those birds
