The genetically engineered lab strain is one that can spread easily among ferrets, meaning other mammals could also be more susceptible.
The National Institutes of Health says revealing all the ingredients could help terrorists trying to make a biological weapon.
But the NIH said it was important for the overall findings to be published in scientific journals because they suggest it may be easier than previously thought for bird flu to mutate on its own and become a greater threat.
"It's very important research," NIH science policy director Dr. Amy Patterson said.
Bird flu, known formally as H5N1 avian influenza, occasionally infects people who have close contact with infected poultry, particularly in parts of Asia. It is highly deadly when it does infect people because it's different from typical human flu bugs.
The concern is that one day it may begin spreading easily between people.
The Associated Press contributed to this report.
