Yes indeed, it appears at least one mass die off of starlings has been explained. The United States Department of Agriculture announced it was responsible for the deaths in Yankton, S.D., reports the Christian Science Monitor.
"The USDA's Wildlife Services Program, which contracts with farmers for bird control, said it used an avicide poison called DRC-1339 to cull a roost of 5,000 birds that were defecating on a farmer's cattle feed across the state line in Nebraska. But officials said the agency had nothing to do with large and dense recent bird kills in Arkansas and LouisianaSeems our friends at USDA Wildlife euthanized four million avians in 2009. But no need to fear, because the project does not cost U.S. taxpayers, the USDA says. In fact, it says the farmers pay the USDA to perform the cullings.
Now if we can just figure out what caused the deaths in Arkansas, Louisiana, Alabama, and Kentucky in the last month.