In Nebraska, Wisconsin, Minnesota and Iowa, National Guard soldiers have an assignment they never expected. They're helping to dispose of tens of millions of dead birds — turkeys and egg-laying chickens that are dying or being killed in order to stop the worst spread of bird flu in American history. Several countries have banned imports of US poultry. But only birds are infected, and chickens raised for their meat aren't involved — so far. We hear about wild-bird migrations, factory farms and a threat to a multi-billion-dollar industry.