The Vaux’s (pronounced “vose”) swift is a seasonal migrant through the skies in our neighbourhood. The flight of this acrobatic bird is remarkable, especially if you’ve been patient (or lucky) enough to witness the controlled, spiralling descent of a large congregation into an impossibly small chimney or old growth tree cavity.
It’s not unlike watching the release of a flock of peace doves, in reverse. Paradoxically, the Vaux’s swifts perform this seemingly dangerous maneuver in order to find peace each night.
How do they manage to do this without injury?
Northern Arizona University professor, Peter Friederici, points at some emerging explanations:
Today, technological innovations, from high-speed photography to computer simulations, have enabled biologists to view and analyze bird flocks as never before. So has a new wave of interest from other scientists, including mathematicians, physicists, even economists. As a result, researchers are closer than ever to really getting inside the mind of the flock.
The British evolutionary biologist William Hamilton, in 1971, coined the term “selfish herd” to describe this phenomenon. Each member of a flock, he wrote, acts out of simple self-interest. When a predator approaches a flock, all the individuals in the group move toward the safest place—namely, the middle of the group—in order to reduce the chances of being captured. Observations of juvenile shorebirds have hinted that it may take them a while to get the hang of this, because they learn to form cohesive congregations only over time. As they do, natural selection dictates that the birds least able to hang with the group are most likely to be caught by predators.
Although, in the case of the Vaux’s swift collective chimney dives, the highly coordinated behaviour is not a reaction to an immediate predatory threat. It seems to simply afford the flock some peace to rest at night (although I can’t help but wonder how the risk of lodging yourself and hundreds of your kin into a narrow, dark chamber at high speed balances out against the risk of roosting in a more exposed, easy to access perch like a tall tree?).
If this acrobatic flocking feat is not necessary, why do Vaux’s swifts do it?
Perhaps it’s because they can and they have fun doing it! And, it may not be a question of why but rather how. The riskiness may be less than it appears if we can decipher their methods.
According to what’s in the literature, it turns out there are only three simple rules to forming tightly cohesive flock (and these apply to all animals that form and move in large groups). Each animal needs to:
- Avoid colliding with its immediate neighbours (called Separation),
- Be generally attracted to others of its kind (called Alignment), and
- To move in the same direction as the rest of the group (called Cohesion).
Apply these methods at high speed and a flock is able to move in a complex pattern of motion and interaction that would be extremely difficult to communicate without a single shared brain.
The crux of this collective act is the individual birds ability to react to the actions of its neighbours 3 or 4 wingspans away. As explained by Ornithologist, Wayne Potts, in order for flocking behaviour to be successful, a wave of motion must propagate at least three times faster than could be explained if the individual birds were just watching their immediate neighbours. This phenomenon was coined the Chorus Line Hypothesis by Potts after the synchronicity required by a large group of dancers.