I'm not a biologist or an earth scientist, but I'm thinking it's something solar/magnetic. Both fish and birds have been documented to have a sense of magnetics, particularly the magnetic north, so perhaps something happened to that. Many of the reports of deaths say the birds were stumbling about and crashing into cars and power lines. Obviously, we don't know the full extent of their magnetic sense, but it could be hardwired in to their brains and difficult to function with change.

Again, I'm only a computer scientist!