The mystery of starling flight patterns

Published: November 12, 2019

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Mesmerizing. Hypnotizing. Pulsating. Those are just some of the words that come to mind if you’re lucky enough to witness a “murmuration” of starlings in the sky. The sight of hundreds, if not thousands of birds, moving as one, undulating, swooping, and diving, is as beautiful as it is compelling, especially since much of what they do, why, and how they’re doing it is still a mystery.

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Let’s start with the “what.” Murmuration is not just the word for the movement. It’s also the name for a flock of starlings (like “murder” is used for crows, and “charm” for finches), and the sounds they make during the act—like the swoosh of blood heard in a heart murmur. Ontario’s resident species is the European starling, introduced to North America in the 1890s. Although many other birds move collectively, few can match the speed and synchronicity of starling aerial manoeuvres.

Now the “why.” Murmurations can only occur if there’s a congregation of starlings, such as when they roost. It’s suspected, but not proven, that the birds roost together for warmth. But since the birds often fly 40 kilometres to join an overnight resting group, likely burning more calories than they’d save by staying put, heat can’t be the singular reason. Another thought is the birds flock together to communicate with each other about feeding sites, as their roosting areas are often close to grain fields or other foraging areas.

Murmurations don’t occur in late spring and summer. “They typically correspond with non-breeding season congregations,” says Stuart A. Mackenzie, the director of migration ecology at Bird Studies Canada. That is, “fall through spring, close to a roosting site. They’re used to avoid predators, or play the safety-in-numbers game.”

But just how do such large numbers of starlings move as one, with such fluidity?

The answers began to come rapidly, oddly enough, after the generation of an application by a computer scientist. In 1987, Craig Reynolds created a computer simulation of a flock of birds intended for graphics and motion picture use. The simulation was extremely similar to an actual murmuration he called “Boids.” Reynolds based his algorithm on three movement rules: separation (steer to avoid crowding or hitting other birds); alignment (steer towards the groups’ average heading); and cohesion (steer towards the average position of the closest birds).

The computer simulation illustrated that modelling could greatly assist researchers. In 2010, Andrea Cavagna, a scientist at the National Council of Research at the University of Rome, used advanced computational modelling and video analysis to prove that flocks aren’t governed by a single individual, but collectively by all flock members. This was named “scale-free correlation.” Then, in 2013, a colleague discovered that the three rules the birds follow to move with such solidarity is garnered from six or seven of their closest neighbours in the group within a fraction of a millisecond. Six or seven, it seems, is the number that achieves the optimum balancing act between individual effort and group cohesion. Like snowflakes on the cusp of an avalanche, the velocity of an individual affects its immediate neighbours.

Science may be able to explain the rules that govern murmuration movement, but there are still unanswered questions. Once a threatened bird makes the initial change in direction, how and why do the others follow suit? “We don’t know yet how a shift happens, or how these flocks are coordinated,” admits Stuart Mackenzie. “It’s a mathematical and biological marvel.”

Perhaps we don’t fully understand exactly how or why murmurations occur. But do we really need to? We can still appreciate the beauty of the mystery. And maybe that’s enough.

 

 

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