Since corn / maize was not available, wheat likely was a major component. Soybeans were also not present. Fish and animal offals would have been logical sources of protein concentrates. Free-range keeping and seasonality of production would also be likely as former would be important for meeting micronutrient needs. Somewhere I read some may have routinely employed broody roosters which would have helped once chicks hatched. I doubt that a given poultry raiser would raise more than a few hundred birds and like raised other animals as well which would help provide feces for birds to glean through.
