A nest is “a disordered stick bomb,” resilient in ways that humans have hardly begun to understand, much less emulate.
Sticks in a cylindrical experiment at the Goodyear Polymer Center at the University of Akron in Ohio, where Hunter King investigates the engineering of birds’ nests.Credit…Andrew Spear for The New York Times
By Siobhan Roberts March 17, 2020
The term “bird’s nest” has come to describe a messy hairdo, tangled fishing line and other unspeakably knotty conundrums. But that does birds an injustice. Their tiny brains, dense with neurons, produce marvels that have long captured scientific interest as naturally selected engineering solutions — yet nests are still not well understood.
One effort to disentangle the structural dynamics of the nest is underway in the sunny yellow lab — the Mechanical Biomimetics and Open Design Lab — of Hunter King, an experimental soft-matter physicist at the University of Akron in Ohio.
“We hypothesize that a bird nest might effectively be a disordered stick bomb, with just enough stored energy to keep it rigid,” Dr. King said. He is the principal investigator of an ongoing study, with a preliminary review paper, “Mechanics of randomly packed filaments — The ‘bird nest’ as meta-material,” recently published in the Journal of Applied Physics. (He added that, obviously, the bird-nest stick bomb never explodes.)
Dr. King and his colleagues seek to answer simple questions: What is the underlying mechanical principle behind the bird nest’s construction strategy? What are the statistically robust characteristics of “the nest state”? That is to say, what separates a nest from the same sticks and twigs collected into a tight bundle or scattered helter-skelter?
“Birds perform what I’ve been calling ‘mechanical synthesis,’” Dr. King said. “Whereas, on a molecular scale, a chemist will synthesize polymers of varying length or stiffness in anticipation of bulk mechanical properties, the bird chooses skinny elements from its environment, with some selection criteria in expectation of nest performance.”
A nest has a certain chemistry — an alchemy, almost. From humble parts, a greater sum emerges and coheres. And, presumably, its generic principle would not be exclusive to nests. Rather, it would be widely applicable to structures in architecture, packaging, shock-absorption and more.
For the study’s nest-building protagonist, Dr. King chose the cardinal, because in building her nest she essentially just shoves sticks together; the robin complicates matters with mud. As Dr. King and his collaborators wrote in the proposal that won them a grant from the National Science Foundation: “When a cardinal builds her iconic cup-nest, she uses her own body as template and molds thin twigs, grass strands, and bark strips into a structure that, despite its softness, reliably holds its shape against various mechanical perturbations.”
In modeling the delicate interplay of the nest geometry, elasticity and friction, Dr. King and a graduate assistant, Nicholas Weiner, fashioned a tabletop experiment with “a little bit of a steampunk style.” They built an artificial nest: a cylinder containing hundreds of bamboo skewers, laser cut and bought in bulk. Then around it they created — from the components of an apparatus previously used to characterize the mechanical response of rubber — a chamber to measure the response of the nest when it was repeatedly compressed.
Although the goal is simple, Dr. King calls the experiment “ugly” because the rich system has an overabundance of factors at play. “This makes the problem not easily amenable to elegant theory or simple analysis,” he said.
Dr. King has been pursuing this line of investigation for several years. As research problems go, it’s a bird’s nest of a bird’s nest: All the input parameters and boundary conditions are interwoven, in ways that prove difficult to tease apart.
More than a metaphor
The paper recently published by Dr. King and his collaborators primarily reviews the field of nest research, such as it is.
One reference on Dr. King’s bookshelf is a classic title by Mike Hansell, “Bird Nests and Construction Behavior.” Dr. Hansell, a professor emeritus of animal architecture at the University of Glasgow, conducted field work in museum collections around the world. At the Natural History Museum in London, for instance, he found “an unremarkable looking cup nest of grass and rootlets” by the now extinct piopio (Turnagra capensis) of New Zealand.
“The bird itself was last observed in 1947,” he wrote. “Possibly no other nest of this species remains in the world. It is an enduring expression of behavior that can no longer be seen. To touch it is to be as close to its maker as to touch a brush stroke of a Van Gogh sunflower.”
In cascading chapters and subsections, Dr. Hansell explored topics like nest shape, decoration and size, and the nest as a factor in mate selection. For the blue-footed booby, nest-building is “reduced to the exaggerated presentation of tiny pieces of vegetation by the male to the female, a trait that might well be expected to be under the influence of sexual selection,” he wrote, with a nod to Charles Darwin.
Over the years, through encounters with architects and engineers, Dr. Hansell has developed “a certain skepticism about what lessons they can learn from bird nests,” he said in an email. “There are several thousand species of nest-building birds; each is trying to create an environment to protect their progeny that is special to their biology and environment. Are there things that we can learn from these structures? There surely must be, but to do that, we must have proper understanding.”
Dr. King is more optimistic. In his paper, he surveyed the array of materials, from the round grains of sand to the slender filaments in cotton balls, that possess emergent properties: When the elements are packed together randomly, they behave collectively, in a process called jamming.
“If we think of the bird-nest material as a bunch of sticks that are just jammed together, which to some very crude degree is accurate, then as a material it would fall somewhere in the spectrum between sand and cotton,” Dr. King said.
As a reference, he pointed to a 2012 paper that explored how heavy-duty Duo-Fast staples, or “u-particles,” cohere and interpenetrate into a clump. He also noted a 2016 paper on “aleatory architecture.” In Latin, alea refers to dice or gambling; the researchers asked whether design could arise from disorder: “Can we develop a vocabulary of concepts to interpret various orderings by chance?”
Easier said than done. The Beijing National Stadium, known as the “Bird’s Nest,” was initially designed to be an accretion of truly randomly placed pieces. But the conceptual goal ultimately failed, owing to engineering restrictions: The structure is a highly ordered 42,000 tons of steel, a mere “monument to a metaphor,” the researchers noted.
There have been more successful literal interpretations. Inspired by snow and sand, Karola Dierichs and Achim Menges, architects at the Institute for Computational Design and Construction at the University of Stuttgart, created starlike particles that are dropped into place to form a nest-esque structure, an example of what they call “aggregate architecture.”
“Stability is established only through random contacts, not local adhesive mechanisms, such as glue or screws,” Dr. Dierichs said.
An asymmetrical energy loop
Of course, a bird’s nest is not entirely random; the builder weaves or places the elements. But what is the universal logic? What is the quintessence of “the nest state”? Like a bird building a nest, Dr. King hopes that a “flexibility in thinking” will “let the underlying story emerge.”
He and his collaborators have explored how the materials in the artificial nest pack together, and how the ensemble absorbs energy. So far, they have observed what Dr. King called “a steady state hysteresis caused by reversible slippage.”
The term “hysteresis” is derived from the ancient Greek, meaning “deficiency” or “lagging behind.” Simply put, it describes how a physical system behaves differently depending on what was done to it previously — the system has a history. Hang two one-pound weights from a rubber band, then remove one. With only one weight remaining, the band will be still stretched farther than if only one weight had been added in the first place. That is hysteresis. The rubber band is not behaving like an ideal spring; there is an energy loss to the system.
Something similar happened with the “nests” in Dr. King’s plexiglass cylinder. The sticks were slowly compressed to maximum stress, and then released, repeatedly. During each cycle, the sticks compressed a little more and then bounced back, but only partway; this was hysteresis happening. Eventually, for any given skinniness of stick — its aspect ratio, diameter divided by length — the system founds its maximum, or steady state, density.
Then the experimenters shmushed the sticks some more, with additional cycles. But their data suggested that hysteresis was still happening. This was unexpected and intriguing; the sticks were at density, and they didn’t seem to be rearranging any further in the shmushing chamber. The team came to call this “steady state hysteresis.”
With computer simulations, they landed on an explanation. The sticks were in fact compressing further, slightly rearranging as one stick slid along another. But this slippage undid itself upon release — “reversible slippage.” The nest became an asymmetric spring: stiff when pushed, soft upon release. (A Polish study in 2018, also influential for this investigation, suggested a similar effect.)
This phenomenon might be exactly what the investigators are after: A process that underlies a nest’s machinations — its fundamental mechanical response under force — and one that should be present in other systems.
These are just preliminary findings, which Dr. King will continue to explore in the lab, and with further simulations by Mattia Gazzola, a mechanical engineer, and his Ph.D. student Yashraj Bhosale, at the University of Illinois at Urbana-Champaign’s National Center for Supercomputing Applications.
Dr. King also aspires to find birds that are willing to collaborate. Recently he attended a bird biology conference in the United Kingdom, in search of advice. A year earlier, he had sequestered cardinals at the Akron Zoo, to monitor them as they built nests, but they showed little interest in the materials he brought from the local arts and crafts store.
After he gave his talk, he was approached by Shoko Sugasawa, a research fellow, and Maria Tellos-Ramos, a postdoctoral researcher, in Sue Healy’s Cognition in the Wild lab at the University of St Andrews. They suggested that zebra finches might be better candidates.
“Zebbies breed readily in captivity and they are happy to use a range of materials including paper strips, cotton strings and coconut fibers,” said Dr. Sugasawa, who is setting up a lab that will film the birds in a breeding cage. “For Hunter’s research, it will be particularly informative to see how different nest materials change the properties of resulting nests.”
Dr. King will also roll the dice again at the zoo, with sticks and fibers of varying flexibility and length, although he’s not counting any nests before they hatch.