I'm not certain this can be blamed in any way on battery chicken farms, however much one dislikes the practice. Battery farms were not about during the time of the Spanish flu epidemic and still the virus mutated. In BC in a recent bird flu outbreak, barns were sealed and the birds gassed.

The virus is well established in the wild fowl population and in BC, most of these farms are in the Fraser River delta which is on a major migratory flight path and winter feeding area. The virus was thought to have been passed to farm stock from the wild bird population. More likely, initially to free range poultry, than those in inclosed buildings.