Starling flocks are magnificent to watch; thousands of birds elegantly dancing through the sky as one in a seemingly perfectly choreographed manner. But how do they fly in such a coordinated way? This has mystified scientists for many years and while numerous hypotheses have been proposed in the past, none seemed to quite fit the bill. Now, within just two weeks of each other, two independent studies have been published which have finally unlocked the secrets behind this intriguing behavior.
Swarming is observed in numerous different animals, from birds and insects to fish and mammals. It’s been suggested that this behavior likely helps to protect the animals from predators. While previous studies have shed light on how birds manage to come together into ordered flocks, little was known about how they select their density, i.e. how far apart the birds stay from each other. Previous theories proposed that information was exchanged locally between neighbors; however, some argued that this was insufficient to explain flock organization.
To find out more, University of Warwick researcher Dan Pearce and colleagues set their eyes on the sky in Brighton, UK, and filmed starling murmurations (flocks). They found that it was never so dense that patches of sky could not be observed through the flock, suggesting that changing patterns of light and dark play a role in flock movement. Furthermore, this also indicated that a bird is always able to see out from inside the flock.
The researchers then used a previously developed computational technique that models how birds fly in the same direction as their neighbors and adapted it to explain how birds may regulate flock density.
As described in PNAS, they found that starlings aim to maintain an optimum density which allows them to gather information on their surroundings. This occurs when the birds are able to see light through the flock at many angles, which is known as marginal opacity. As the birds self-organize in an attempt to achieve this ideal density, patterns of light and dark are produced that deliver information to the flocking birds.
A dynamic and changing silhouette is therefore created by the birds altering the positions and angles at which they fly, which subsequently alters the amount of light that can penetrate the flock. The more complex the silhouette, the more information about the flock it contains.
Further insight into flocking behavior has also been revealed recently by a separate group of researchers, which has been published in Nature Physics. Rather than looking at flock densities, these researchers looked at how starlings in flocks align and initiate turns. Information about direction changes spreads through the flock like Chinese whispers without a loss of information, which is critical to prevent the flock from dispersing.
To achieve this efficient information transfer, they found that rather than copying the direction in which a neighbor is flying, birds copy how sharply their neighbors turn. Information on direction can therefore propagate through the flock at a constant speed, which was calculated to be around 20-40 meters per second. This rapid transmission of turning information allows the birds to respond in as little as half a second without breaking apart.
Check out this video of a model from Warwick which shows how even predators can’t break the flock apart:
[Header image "Starlings over the West Pier," by Ian Usher, via Flickr, used in accordance with CC BY-NC-SA 2.0]