... As I understand it, the authorities have all but concluded the birds were murmurating to flee a predator, they dove en masse toward the ground as an evasive maneuver, and some didn't "pull out" of the dive. ...
I found an orphaned post from 2003 describing a mass suicide by starlings in Stuttgart, in which a mass of the birds nose-dived into the ground (with multiple witnesses). It's now been transferred into this thread:
More than 1,000 birds were found dead near a Missouri town on Monday and officials want to know what happened.
The Missouri Department of Conservation received reports of large numbers of dead birds found near Sikeston and went to investigate.
Sikeston is located in southeast Missouri.
Biologist Kevin Brunke found more than 1,000 dead birds — red-winged blackbirds, brown-headed cowbirds, grackles and European starlings — in fields and neighborhoods near Wakefield Road, the post said.
They also found several injured birds in vegetation near the roadway.
Video of the scene can be found here but may be disturbing to some viewers.
Mothershead wrote that the conservation department had received calls about dead and injured birds between 8 and 9 p.m. on Sunday — about the time a thunderstorm rolled into the area, according to the post.
One resident reported “hearing birds [hit] the ground” near her home, the post said.
Mothershead and Brunke think it’s likely the flock got caught up in the storm. “... it’s reasonable to conclude that the flock spooked during the weather event, and were caught up in high winds or lightning,” Mothershead wrote. “As birds couldn’t recover in flight, many fell to the ground and perished or became injured.”