I just read this morning an article in the on-line addition of the Minneapolis Star-Tribune. The following quote I thought was interesting - apparently in Minnesota, large masses of birds do die from disease - although in this case it is still a mystery as to why they died.
"The birds that died -- red-winged blackbirds, common grackles, brown-headed cowbirds and European starlings -- are abundant species that flock together. Roosts can contain tens of thousands to more than 20 million birds, Audubon said. And that makes them more vulnerable to disease and to an incident that might spook them, said Carrol Henderson, the nongame wildlife program supervisor for the Minnesota Department of Natural Resources.
The U.S. Geological Service lists about 90 mass wildlife deaths from June through Dec. 12. Five list deaths of at least 1,000 birds and another 12 show at least 500 dead birds. The largest was near Houston, Minn., where about 4,000 water birds died between Sept. 6 and Nov. 26 from various parasites. Henderson said Minnesota typically sees one or two large bird kills a year, mostly from botulism, toxic algae, West Nile or some other avian disease."