From Georgina Rannard at BBC News:

Jackdaws use a “democratic” process to decide when to leave their roosts en masse, scientists have discovered. Thousands of jackdaws can suddenly take to the morning skies in winter, creating a whirling black cloud of creatures.

Researchers have now found that the birds call out when they want to leave.

Then when the noise reaches a critical level, it signals the roost is ready to depart, and the birds fly away. It’s a rare insight into how animals make group decisions, Alex Thornton, professor of cognitive evolution at University of Exeter, told BBC News.