The redwing blackbirds that died are warm weather birds that migrate south in the winter. In the spring, suddenly they're all around establishing their territories in marshy areas then just as suddenly in the fall they're gone.
If those birds that died did happen to get caught in the severe cold fronts that have been plaguing the midwest then it's understandable that their systems were shocked enough to kill them.....
And if they got caught aloft in such a system and fell to the ground, they would suffer physical injuries as well.