Here's a list of the major cases. Here is a Google Map. From National Geographic, here is a responsible account. Loud noises, fireworks, crashes, and an availability cascade seem to be the major hypotheses under consideration. Cold weather and poisoning and hypoxia have been cited as well. Don't forget the dead fishes and crabs; the world seems slightly more sub-Malthusian than before.
Here is Twitter on #deadbirds. In Germany and Schweiz they are talking about "Vogelsterben" and fog; Schweiz had some previous cases over the last few years. Here is a 2009 report from Western Australia, no one seemed to pay heed at the time. There was talk of poison pesticides. Here are many other reports of group animal deaths.
This guy thinks it is all a strategy to scare people.
I'll chalk it up to two or three different causes, combined with coincidental clustering. Is there betting on how many more cases will surface?