Lakeside, California— Hilliker's Ranch Fresh Eggs in this San Diego suburb has 30,000 chickens in three "cage-free," open-air barns, where birds crowd the floor like rush-hour riders on a big city subway. "A cage-free aviary is a very interesting science experiment," says Frank Hilliker, who runs the farm his grandfather started in 1942. He worries mightily about infections spreading through the massed birds. On his iPhone, he pulls up a list of the vaccines his chickens get: against Newcastle disease, infectious laryngotracheitis, coryza, colibacillosis, salmonella, infectious bronchitis, and fowlpox. There is one disease for which his chickens have not been vaccinated: a highly pathogenic avian influenza (HPAI) that's now racing around the world, killing 90% or more of the poultry it infects. Endemic in migratory birds, this HPAI emerged in U.S. poultry in February 2022, and to date has killed or required the culling of a record 58 million chickens, turk...