The Science of Starling Flocks: Birds as Fluid

Starlings are an invasive bird species that congregate in flocks of up to a million members. In mass, the group acts as a cohesive unit to protect itself against predators. When perturbed by a threat, the cloud of animals writhes apart, only to congregate seconds later like an elastic glob of rubber. The mechanisms behind the fluid motion of starling flocks continue to perplex scientists, but help us understand how groups of animals move in unison and make collective decisions.

Starling flocks murmuration

Individual starlings can only keep track of six to seven peers at once. Scientists originally thought that each bird studied the direction and speed of its neighbors to make a decision about its own flight path. More recent studies have unearthed that starling flocks operate more like a fluid network, where each bird consolidates information from adjacent birds, and the flock as a whole.

Close-up of starling

Starling flocks appear to murmur and pulse because the group does not have a uniform density. Parisi et al found that individual birds maintain interaction with one another based on how many birds are in between them, rather than how far apart they are. This means that the flock can maintain communication at different densities, even when the mass is split apart by a predator. The varying density inside a flock produces varying levels of visibility for each bird. Researcher Daniel Pearce found that individual birds make flight path decisions based on how much of the sky they can see from inside the flock. His team constructed computer models where individuals tended to migrate toward a certain shade of darkness. The animations resemble bird flocks, schools of fish, and swarms of insects, depending on the parameters in the simulation. He concluded that a flock’s ‘patchiness’ makes the mass more flexible amidst perturbations.

In most bird flocks, the decision-making impact between individuals varies depends on how much they can see outside the flock. This non-uniform field exists in other types of animal groups, such as navigating wolves that rely on a dominant individual to carry the most impact. Similarly, a hungry animal influences direction more than a well-fed member of the pack. In the case of starlings, the aggregate decision to move away from a bird of prey is informed mostly by the birds who see the approaching enemy.

Starling flock models.

Physicists have adapted their own tools of analysis to the starling flock quandary. In recent years, researchers have used equations from particle physics to describe bird flocks as a material that undergoes state changes, like melting metal. Andrea Cavagna and Asja Jelic used video footage and motion tracking software to analyze starlings outside a train station in Rome. They discovered that information travels through the flock at a constant rate, instead of dissipating from neighbor-to-neighbor. This requires all individuals to undergo a change of state at once, and allows the group to act as a cohesive material at astronomical numbers. The information about a flock’s surroundings integrates faster when the flock is more tightly aligned, reinforcing the evolutionary pressure of cohesion against potential predators.

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