http://www.cdc.gov/flu/avian/gen-info/spread.htm
Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
Avian influenza viruses circulate among birds worldwide. Certain birds, particularly water birds, act as hosts for influenza viruses by carrying the virus in their intestines and shedding it. This fact sheet discusses spread of avian influenza viruses among birds, including wild and domesticated birds.