The movement of flocks of birds is an incredibly coordinated phenomena. Appearing to move as if a single being, flocks seem to instantaneously twist and turn in the air. A massive, swirling flock of birds comes together and breaks apart again, seemingly without any chaos or disorganization. So, why do they do it?
The proverb “birds of a feather flock together” has a lot of truth in it. Birds of a species frequently form flocks, as protection from predators. Ornithologists explain this as a tactic for ‘safety in numbers.’ After all, in a large group, an individual bird is much less likely to become a predator's meal. It makes it much more difficult to pick out one specific target. Trying to focus on one bird when an entire flock is moving in ever-shifting waves would be a feat indeed. Therefore, especially when it’s time to migrate, it is much safer to fly as part of a flock then alone. "If you are faced with food scarcity, you have two options," said W. Alice Boyle, an adjunct lecturer in UA's department of ecology and co-author of a study published in the March 2007 issue of American Naturalist. "You can either forage with other birds, or you can migrate." Also, it is beneficial to be part of a flock when foraging for food because each bird is essentially exploiting the eyes of every other bird in the flock.
According to a University of Illinois study published in the July 2008 issue of the journal Integrative and Comparative Biology, many birds migrate at night because the air is cooler and calmer, and there are fewer predators, yet they continue to fly in tandem even when they are 250 meters or more apart. How do they do this? Well, many birds fly in a V-formation, helping the individuals to conserve energy, lessening air resistance. Through this coordinated spacing, the drag of every bird in the formation is reduce, and the individual’s position is optimized. However, not all birds are working equally. The bird leading is actually working the hardest because it is flying in undisturbed air, but it improves the aerodynamics of the two following birds through the upwash it creates.
In flocks, individual birds coordinate movement and prevent from colliding with each other through something known as swarm intelligence. According to a March, 2000, publication by Yang Liu and Kevin M. Passino from Ohio State University’s Dept. of Electrical Engineering, swarm intelligence is “the emergent collective intelligence of groups of simple autonomous agents.” With an autonomous agent being a “subsystem that interacts with its environment, which probably consists of other agents, but acts relatively independently from all other agents.” For example, an individual bird in a flock is not following any leader bird’s orders whilst flying. Yet, birds manage to fly in such a coordinated manner through reacting and adjusting their movements fit with their neighbors. With each bird that makes up the flock flying in sync with its neighbor, and focusing on not colliding with them, there does not need to be a leader to orchestrate movements. Therefore, the collective swooping and swerving dance of a flock is not the property of any individual but rather is a property of the group.
Research by Wayne Potts, published in the journal Nature in May, 1984, helped explain how flock movements are initiated and coordinated. It was initially thought that the execution of abrupt maneuvers by flocks was initiated by “though transference” or “elecromagnetic communication” but Potts, through a frame-by-frame analysis of high-speed film of sandpiper flocks, found that any individual can initiate a flock movement. This movement then propagates through the flock in a wave radiating out from the initiation site, in a similar manner to a human chorus line. These "maneuver waves" were initiated by visual communication and could move in any direction through the flock, including from back to front. But, the flock usually only responded to birds that banked into the flock, rather than away from it. Since birds turning away from the flock run the risk of being separated from it and getting picked off by the predator, others will not follow them. This rule, besides the apparent benefit to individuals, helps prevent indecision by the flock and permits it to respond rapidly to attack.
The reaction time of individual birds varied greatly from the general reaction time of the flock. In a controlled laboratory setting, the mean reaction time by an individual bird to a light flash was 38 milliseconds. However, these maneuver waves spread through entire flocks in less than 15 milliseconds (although the first bird to react to an initiator took 67 milliseconds). The conclusion that Potts drew from this was that the farther the birds were from the initiation site, the sooner they were able to prepare to react to their neighbor because they could see the wave approaching them. This is where the chorus line analogy comes in - the Rockettes at Radio City Music Hall can see and therefore anticipate an approaching high leg kick while it is still well down the line. When rehearsed maneuvers of human chorus lines were filmed, and a kick was unexpectedly initiated, it propagated down the line at less that 108 milliseconds. Human visual reaction time is 194 milliseconds, so that is almost twice as fast.
An incredible video of a murmuration of starlings (yes, technically that’s what a flock of starlings is called) was filmed in Canada swirling for several minutes. I recommend viewing the video in full screen: