In Eyrie, the goliath birds come in several flavors. The largest variety inhabit the frozen taiga, preying upon chickens, monitors, and anything they can get their beak around. When times are lean, they raid the weald and kjarr.
Their cousins, the domesticated goliath birds raised on the Crackling Sea Eyrie ranches, came from further north and were brought south because of their temperament. The Redwood Valley Eyrie had several attempts to create their own goliath bird ranches, efforts which ended in failure when the birds escaped into the aneda and redwood forests.
Eyrie’s version of goliath birds is based on three different large, flightless birds that are now extinct. The domesticated goliath birds that the Crackling Sea opinici raise are modeled after the elephant bird and moa. The feral goliath birds of the taiga are based off of the terror bird.
The real life story of the terror bird is just as interesting. They were considered excellent predators that ruled the plains until 17,000 years ago. During the Great American Interchange, terror birds flowed north from South American into North America. They were frightening predators reaching almost 10 feet tall. It’s worth reading about them over on Wikipedia under the heading Phorusrhacidae. BBC Earth also has an exciting article on them.