Here's a 2007 thread from the Fortean Times discussion board:
http://www.forteantimes.com/forum/viewtopic.php?p=1055926&sid=3974caa31996a1dbf2d65da4cb6f5b2eThey were discussing an incident in Australia that had been linked to lead dust from a mining operation, but then there was a second incident elsewhere with no clear explanation.
On edit: Here's another case from New Jersey in January 2009. The full story is only available from the paper's archive -- but from the headline, it sounds like the state government's claim that it was a deliberate culling program was not a definitive answer.
http://www.nj.com/news/ledger/jersey/index.ssf?/base/news-12/1232947558327210.xml&coll=1Dead birds by the hundreds, but few answers
Ten-year-old Skylar Venus was Rollerblading in her Franklin Township neighborhood over the weekend when she noticed a dead bird lying on the road. Then she saw another one. . . .
The Venuses' neighbors in the southern part of the township in Somerset County were having similar experiences beginning Friday night. Franklin residents said they believe hundreds of dead birds fell onto lawns and roads.
Standing in her Park Lane driveway, beside a dead bird, Venus pointed yesterday afternoon to houses where black birds with yellow beaks also lay dead. One neighbor even told her he had seen birds falling straight from the sky, their wings still as they came down like rocks. . . .
Last night, a spokeswoman for the New Jersey Department of Health and Senior Services confirmed the dead birds were part of a USDA program to reduce the European starling population. Donna Leusner said the state health department was not part of the culling program but had been notified of plans to feed the birds a "controlled substance."...