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How this disease advances is rather interesting. It does not just move west or east. Birds carry it south where they infect other birds. These second birds return north next season, but a little more west. The next year those birds go south and infect more bird who, in turn, go back north but a little more west. IOW the disease tends to move north and south significantly while moving slowly east or west.
Activity has intensified in understanding bird migration patterns.
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