You've probably seen murmurations with starlings.....they form up in very dense flocks that seem to move in unison.
The problem is that most of the birds in the flock can only see birds immediately in front of them.....not the ground. They turn based only on the birds in front of them.
Sometimes a predator will spook the lead birds.....they have room to pull up, but the birds behind them are a fraction of a second behind them, and they and the rest of the flock start smashing into the ground.
Watch the video carefully......most of the birds get up and fly away.
It appears to be a 'murmuration' gone bad.
https://www.cnn.com/2022/02/18/world/blackbird-swarm-crash-mexico-scn/index.html
You've probably seen murmurations with starlings.....they form up in very dense flocks that seem to move in unison.
The problem is that most of the birds in the flock can only see birds immediately in front of them.....not the ground. They turn based only on the birds in front of them.
Sometimes a predator will spook the lead birds.....they have room to pull up, but the birds behind them are a fraction of a second behind them, and they and the rest of the flock start smashing into the ground.