Supporters of factory farming often point out that bird flu can be spread by free-range flocks, or by wild ducks and other migrating birds, who may join the free-range birds to feed with them or drop their feces while flying overhead. But, as Brown has pointed out, viruses found in wild birds are generally not very dangerous.
工厂式养殖的支持者常常指出：自由放养的家禽也能够传播禽流感，不仅如此，野鸭和其它候鸟与放养家禽争食或飞过时排出粪便也有可能传播这种疾病。但正如布朗指出的那样，野鸟身上发现的病毒一般危险性不大。