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Collective Motion Multiplies Design Requirements


A memorable sequence from Illustra Media’s documentary Flight: The Genius of Birds examines the phenomenon of starling murmurations (see it here). When you consider the training required for six fighter pilots to fly in formation, it becomes all the more remarkable to watch half a million birds perform split-second maneuvers in close proximity to one another.

In the film, European scientists sought to understand the birds’ collective motion by plotting the positions over time of individuals and small groups of birds within the flock. Now in a new paper in PLOS ONE, four UK scientists try a different approach. They monitored a flock of “citizen scientists” who volunteered to record observations of starling murmurations over a two-year period. Some 3,000 volunteers from 23 countries participated. The large data set, mostly gathered within the UK, allowed the researchers to address little-understood questions about this spectacular example of collective motion, such as seasonal activity, dependence on temperature, and whether or not predators affect the size or length of a murmuration. Here’s a quick summary of the findings:

  • Flock sizes increased from October to February, then declined.
  • Average duration was 26 minutes; longest ones were at the beginning of the season.
  • Cool temperatures weakly increased murmuration durations, but day length was more significant.
  • Predators were observed in only about 30 percent of the murmurations.
  • When predators were present, the birds tended to descend en masse to their roosts rather than disperse.

Based on the data, the authors believe that predator avoidance (the “safer together” hypothesis) is probably more in play than temperature (the “warmer together” hypothesis):

[O]ur findings suggest that the collective behaviour observed in starling murmurations is primarily an anti-predator adaptation rather than a way of attracting larger numbers of individuals to a roost for warmth. Suitable roosting sites attract large numbers of birds who would be vulnerable flying to the roost individually. Murmurating above the roosting site provides multiple advantages in terms of the dilution effect, increased vigilance leading to the detection effect and predator confusion. This model of murmuration relies on having a critical mass of birds arriving at more-or-less the same time to initiate the murmuration and further study of the behaviour of starlings at the start of the murmuration (and indeed, just before the start of the murmuration) would be valuable in unravelling how this behaviour develops from a relatively few number of individuals into a spectacular collective behaviour comprising potentially tens of thousands of individuals.

Discovering one reason for a behavior, however, does not negate other possibilities. Perhaps the birds sleep better after an energetic exercise program. Or, maybe it gives them pleasure somehow. Predator avoidance may just be a side benefit, since predators were not observed during most of the events. It seems overly costly to evolve this kind of elaborate flight behavior for predator avoidance when simpler options could do, such as camouflage or scattering. And why didn’t the predators evolve counter-measures, like engaging in attack murmurations of their own, dive-bombing the flock en masse in their roosts? Have hawks been fooled by the starlings’ trick for millions of years? For these and other reasons, evolutionary explanations fall short. The authors don’t even mention evolution or speculate about how the behavior arose.

One thing we can be sure of: performing split-second decisions in tight formation in 3-D without colliding doesn’t just happen. To do what these birds do takes precision flight hardware and software. We appreciate the effort of the researchers and the citizen scientists to gather all this data. It does provide new insight into a marvelous natural wonder. The most important questions, though, remain unanswered by those who restrict their explanations to methodological naturalism.

Collective behavior is seen throughout the animal kingdom: in swarming insects, shoaling fish, stampeding mammals, and flocking birds. The phenomenon is so interesting to the Human Frontiers Science Programme (supported by 15 countries including the United States) that it recently awarded $1 million to a team led by Dr. Alex Thornton to study it. News from the University of Exeter says, “The riddle of how these often vast numbers of individuals synchronize their movements so flawlessly as to behave almost as a single being has only recently begun to be unravelled.”

Thornton is particularly interested in how individual characteristics affect the group, since no two individuals are exactly alike. Even human “flocks” cross the divide between individual and group behavior, as seen in traffic flow and crowd dynamics (for example, doing “the wave” at a baseball game). For the next three years, Thornton’s team will study intelligent members of the crow family, jackdaws and rooks, which often flock together.

Dr Thornton added: “Although people may not realise it, the familiar sight of flocks of jackdaws and rooks that darken our winter skies is amongst the most complex aggregations of animals on Earth. By studying the movements of individual birds within flocks, and their interactions with one another, we will help to reveal how complex societies remain cohesive and make collective decisions.”

Large aquariums delight visitors with their displays that often include swarms of anchovies swimming like one giant organism, all turning on cue. A new paper in Science Advances (an open-access journal of the AAAS) seeks to understand “the effects of external cues on individual and collective behavior of shoaling fish.” What happens when you scare a school of fish, or attract them with food?

To date, experimental work has focused on collective behavior within a single, stable context. We examine the individual and collective behavior of a schooling fish species, the x-ray tetra (Pristella maxillaris), identifying their response to changes in context produced by food cues or conspecific alarm cues. Fish exposed to alarm cues show pronounced, broad-ranging changes of behavior, including reducing speed and predictability in their movements. Alarmed fish also alter their responses to other group members, including enacting a smaller zone of repulsion and increasing their frequency of observation of, and responsiveness to, near neighbors. Fish subject to food cues increased speed as a function of neighbor positions and reduced encounter frequency with near neighbors. Overall, changes in individual behavior and the interactions among individuals in response to external cues coincide with changes in group-level patterns, providing insight into the adaptability of behavior to changes in context and interrelationship between local interactions and global patterns in collective behavior.

Those reactions don’t sound surprising, since we humans can probably relate to watching our neighbors more closely when alarmed, or rushing past them to get free stuff. So again, while one appreciates the graphs and charts of relative speeds of the fish when they are subjected to external cues, the paper leaves the most interesting questions unaddressed: how could evolution equip fish with the hardware and software to respond quickly in coordinated fashion while swimming millimeters apart?

The authors mention “rules of interaction,” but who made the rules, and who enforces them? How did the fish learn the exceptions, when the rules become “context-dependent”? If every individual did not know the rules, frightened fish might make like the Midianites in the story from the Book of Judges, killing each other off in the confusion of the moment. Starlings appear to follow simple rules, but without reliable programming in each individual, the murmuration could turn into a demolition derby.

Fighter pilots mastering formation flight require many hours of sophisticated training in intelligently designed aircraft. From this fact, we can deduce that intelligence was involved in the origin of collective behavior in animals. Tellingly, this paper, like the other one, doesn’t get into evolution. It makes you wonder about that claim that nothing in biology makes sense without it.

Photo: Starling murmuration, courtesy of Illustra Media.