How Do Bird Flocks Regulate Their Speed?

Its approaching fall here in Alaska.  This means that birds are getting ready to head south to places where it is warmer so they don't freeze to death during the winter. When birds fly as a group or flock, moving in what seems to be unison and its referred to as a murmuration. 

Scientists wondered how individual birds could fly together in a flock maintaining proper distance and speed so they fly as a group.  Consequently, scientists created a mathematical model to explain how birds regulate their speed and position when flying together as a group. At the end, they did more than just create the model, they actually took time to compare the model of flight against videos of real flocks to check the accuracy of their results.

There have been previous studies which looked at how flocks maintain their shape even during sudden directional changes, and how individual birds maintain speed using a linear model but they don't explain how the birds influence each other over the long distances flocks travel. In fact, many of the previous models assumed that birds mimicked the behavior of their neighbor so they all moved at the same speed. In addition, many of the earlier models did not address the individual fluctuations properly.   

To begin with, scientists chose to model the flight behavior of starlings because they have been studied so they had specific data.  For instance, when starlings fly individually, they maintain a speed of between 8 and 18 meters a second but when they fly in a flock, they fly at 12 meters per second.  They've also discovered that no matter how large the flock becomes, the individual birds are able to change their individual speeds to stay at the group speed.

The model they designed ignores small variations in speed while suppressing large ones so they can get a more accurate results.  Specifically, they looked at statistical field theory which is a framework that describes phase transitions.  They decided that all birds have a property referred to as spin, similar to the spin of elementary particles in physics. They concluded that when the birds match their spin with one another, they maintain the total spin of the flock.

They've also figured out that the group turns when a small group began to turn and the information of the turn spreads through the whole flock and it only takes a second or so for the information to make its way to all the birds.  Thus birds do not copy change in direction, they copy the angle of the turn so flocks turn at a constant speed.  The movement is begun by a few birds, and the information spreads through the flock within a second or less, depending on its size. 

In addition, they applied the model different sized flocks ranging in size from 10 to 3,000 members to see what actions the birds took while flying as a flock.  Once they had these theoretical results, they then analyzed individual flight paths within flocks so they'd have trajectories they could compare with the results of their models and the two groups were quite similar.

Although this was only applied to the starlings, it is believed that this model can be applied to other species of birds since many birds seem to exhibit the same movement when flying as a flock.  They hope to apply the model to other types of birds to see if the model holds true.  Let me know what you think, I'd love to hear.  Have a great day.