The discovery of  possible bird flu cases affecting chickens in north Alabama is under investigation.     Three suspected cases in Madison, Jackson, and Lauderdale counties have been sent off for further testing.   Dr. Joe Hess -- a poultry scientist with the Alabama Cooperative Extension System -- says there is not a food safety issue.  "Chicken and eggs are safe to eat, and the average, everyday consumer doesn't have to worry about avian influenza, from that standpoint," Hess said.       There are concerns the situation could threaten Alabama's $15 billion dollar poultry industry.