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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 ´óÏó´«Ã½ News.

"When a bird calls, it's casting a vote or signalling it wants to leave," Prof Thornton explains. The collective decision to depart then rests on two things.

The first is noise volume and the second is the crescendo or how rapidly the noise levels increase.

Once the birds reach consensus, the roost of thousands launches from the tree within five seconds on average, forming one of the famous winter UK spectacles.

When the noise levels rise more rapidly, the roost leaves earlier, the researchers say.

In Norfolk roosts of 40,000 jackdaws have been observed leaving trees en masse.

Jackdaws want to leave the trees together because it protects them against predators or is useful for "information-sharing", Prof Thornton suggests.

"If you're flying off altogether, you might notice that another individual is particularly well-fed or you can tell from their calls that they've eaten. You might realise that's a bird worth following to find a good place for a meal," Prof Thornton explains.

To observe the birds' behaviour, scientists attached audio recorders to trees where jackdaws roost in Cornwall over two winters.

Led by Masters student Alex Dibnah, the researchers analysed the sounds and compared the noise to times at which the birds left the trees.

To test their findings, the scientists played back the recordings at jackdaws and observed that the roosts responded to the sounds, departing six minutes earlier on average.

When wind noises were played instead, the birds did not leave, indicating that jackdaws responded specifically to calls rather than simply noise disturbance.

Other birds may behave similarly but scientists haven't yet studied it in detail, Prof Thornton explains.

The findings will help scientists understand the effect of human activities on animal populations, he says.

People are increasingly creating disturbances to birds through light and noise pollution, which may be interfering with animals ability to communicate with each other.

"Imagine a big roost near a town or busy road. If the birds can't hear other other and can't form a consensus to leave together, it could have big impacts on their population," Prof Thornton explains.

The research is published in the journal Current Biology.