Migratory birds caused the initial outbreak of avian influenza, but other factors have helped spread the virus, a report from the U.S. Department of Agriculture (USDA) confirmed last week.

The USDA’s Animal and Plant Health Inspection Service (APHIS) has been conducting investigations for the past several months, trying to pinpoint the transmission pathways of highly pathogenic avian influenza (HPAI).

The report spanned more than 80 commercial poultry facilities and included analyses conducted with help from academic, federal, state and industry...