Avian botulism returned to Lake Michigan this year, killing more species and lasting longer than other recent outbreaks, according to state wildlife officials and researchers. The increase came after a two-year lull. The outbreaks first hit Sleeping Bear Dunes National Lakeshore in Benzie and Leelanau counties in 2006. The die-offs rapidly spread across northern Lake Michigan shorelines and killed an estimated 8,000 to 10,000 birds in 2007.