This. They were eating zombie cicadas.
That was the previous summer, and the “experts” said they ruled out the cicadas. Or so they said.
It made me pretty mad that they never said what it was, just that the birds had stopped dying off. I think the bird food stores were going under around here. They were barely hanging on due to the lockdowns. The dying off was pretty well timed to the emergence of the cicadas though. Bet the “experts” just weren’t thorough, though.