Apparently the red-winged black bird is not rare and 3,000 dead birds out of a flock of millions would have been a partial, but not total flock kill. Probably weather related:
http://www.allaboutbirds.org/guide/Red-winged_Blackbird/lifehistory
Winter flocks can be congregations of several million birds, including other blackbird species and starlings. Each morning the roosts spread out, traveling as far as 50 miles to feed, then re-forming at night.
I was guessing a sudden cold snap would do this -
it’s what caused a massive fish kill in Fla a year ago.