Supposedly, the birds were killed by fireworks. That's not the official cause, but they say it's the most likely cause.
I find that extremely hard to believe. Thousands of birds city-wide fall out of the sky at almost the same moment? Not likely... plus, from what I understand there were no gaming wounds in any of them.
The birds apparently died from blunt force trauma. Massive Trauma was discovered in their breast tissue, along with internal bleeding, but that was mostly likely caused by their sudden fall from the sky.
Comments
maybe the residue left in the air by the fireworks was inhaled by the birds and it poisoned them; or cause their lungs to burn to a crisp or whatever