Road repair? Some bird are very sensitive to fumes
Poisoned seed? Some seeds have a coating on them to prevent vermin from eating them. It disolves away once planted but if someone tossed a bag out it could possibly harm the birds.
And then of course....
“I’m not saying it was avians... But it was avians.”
There are also some plants that become toxic after a frost. I think sorghum is one of them.
This is interesting. When I have seen this, it is migratory birds that have fed heavily on insects that were poisoned, usually south of the developed world.
These are starlings, certainly migratory, but they are traveling south, this time of year. When a bird is fattening up before migrating, poisons can get stored in their fat layers. Then during a migration, they metabolize the fats and get a lethal dose. They just fall out of the sky.
The timing of this suggests that they were feeding on poisoned insects, to the north, before they flew south.