EUGENE, Ore. -- The population of endangered Fender's blue butterflies has doubled at Fern Ridge Reservoir since last year, according to a U.S. Army Corps of Engineers survey. The Corps' 2012 estimate of 3,769 Fender's blue butterflies at 11 sites near the reservoir west of Eugene, Ore., is the species' largest known population and continues a trend of positive population growth since it was first detected on Corps lands in 1998.