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Bird Nest Mechanics

November 7, 2022

I was born and raised in a mid-sized city in Upstate New York populated almost evenly by first and subsequent generation Italians and Poles. As an illustration of the proverb, birds of a feather flock together, the Italians generally lived on the east side of the city, while the Polish lived on the west. My brothers and I had a Polish mother and an Italian father; so, we were fed a variety of Italian and Polish foods. I never had any Chinese food until I was away at college.

Scientists are constantly creating theories in order to understand their world; so, when I first heard about the Chinese delicacy, bird's nest soup, I was certain that this was a culinary simulation of a bird nest created using rice noodles dropped into a bowl of soup. How surprised I was to learn that the soup is made from actual bird's nests formed by swiftlets from their solidified saliva. While this seems unappetizing to Westerners, there's a close analogy here to honey, a product of honey bee regurgitation.

Culinary bird nests -Rice noodles and knafeh

Culinary bird nests - Rice noodles (left) and knafeh (right). Rice noodles can be arranged to appear as a bird nest, but knafeh, a Middle Eastern dessert made using the spun pastry, kataifi, is a closer simulation. Greek Baklava is sometimes made to have the same appearance as knafeh. (Left image, Stir-fried rice noodles in Nanchang by N509FZ. Right image, Knafeh made by Syrian immigrants in Argentina by Carlos Disogra. Both images from Wikimedia Commons. Click for larger image.)

Many stereotypes have be reinforced through cartoons, and this is true for our perception of how a bird nest should appear. While birds have nests in everything from tree hollows to sand and soil pits, the popular ideal of a bird nest is the cup nest. These are the nests of the Passeriformes, or perching birds, which encompass the majority of all bird species, and this nest type is the layout of Big Bird's nest. Cup nests are made from flexible materials such as grass and twigs that offer safety and thermal insulation for eggs and fledglings.

A plethora of experiments come into a scientist's mind upon reading the preceding paragraph. How thermally insulating are bird nests? What is the mechanical strength of a bird nest? How do these vary between species and geographical location? Will global warming be a factor in the quality and abundance of nest materials, and will the diminished quality of nests be a factor in bird species extinction?

A team of scientists and engineers have published a paper describing their research on the mechanical plasticity and hysteresis of nest-like structures.[1-3] The team members are from the University of Illinois at Urbana-Champaign (Urbana, Illinois) and the University of Akron (Akron, Ohio). The nest-like structures were random packing of unbonded, semi-flexible fibers.[1-2] Their experiments used bamboo sticks to simulate twig-based bird nests, and they developed computer simulations of nest mechanics.[1-2]

Granular materials exhibit unique behaviors, and it's suggested that nest-like structures are a type of granular material intermediate between random ellipsoidal grains, such as rice, and tangled fibers.[3] The nest-like structures are less rigid than rice, but more rigid than tangled fibers.[3] This study found that nest-like structures are elastic with a non-linear stress-strain curve.[1-3] The stress-strain curve also exhibits hysteresis, a feature likewise found in the compaction of low aspect ratio ellipsoidal particles (e.g., rice) and indicative of energy loss from frictional rubbing between the nest elements.[3] The hysteresis was also repeatable and velocity independent.[1-2]

Nest mechanics experimental setup

The nest mechanics experimental setup.

The rods were 76 millimeter (3 inches) long with a diameter of 2.45 millimeter (0.1 inch) and made from bamboo. The aspect ratio of the rods was 31; that is, the rods were thirty-one times longer than wide.

The cylindrical container was 140 millimeter in diameter.

(Fig. 1(b) of Ref. 2)[2]

The experiments were on 76 mm long bamboo rods with an aspect ratio of 31; that is, the rods were thirty-one times longer than wide.[1-3] In their experiments, the research team used computer-assisted X-ray tomography to create three-dimensional maps of the points of contact between the rods as they were cycled through compaction.[3] It was found that the distribution of contact points between the nest elements was a determining parameter in the mechanical properties of the nest.[3] Compression increased the contact points, and the contact points restrict further bending. As a consequence, increased compaction leads to a stiffer nest, and this causes the non-linear stress-strain curve.[1-3] Overcoming static friction during release from compaction causes an asymmetry in the hysteresis loop.[3]

The experiment differs from an actual nest in one important way. Nests are not confined by a container. As seen in the figures, the container diameter is not much larger than the rod length. Paper author, Hunter King of the University of Akron says that the team has planned subsequent experiments in which they hope to extrapolate the mechanical behavior to that of an infinite nest, but there's still the question of whether a containerless nest of untangled rods would be self-supporting.[3] Even if containers are required, nest-like filling of mechanical structures might have useful applications.[3]

Experimental stress-strain curves and simulated response.

Experimental stress-strain curves (gray) and simulated response (red) for quasistatic stress-strain cycles.

Considerable hysteresis is evident (The basis for a novel mechanical heater?).

(Click for larger image.)


  1. Yashraj Bhosale, Nicholas Weiner, Alex Butler, Seung Hyun Kim, Mattia Gazzola, and Hunter King, "Micromechanical Origin of Plasticity and Hysteresis in Nestlike Packings," Phys. Rev. Lett., vol. 128, no. 19 (May 13, 2022), Article no. 198003, DOI:https://doi.org/10.1103/PhysRevLett.128.198003.
  2. arXiv version of ref. 1, December 1, 2021.
  3. Philip Ball, "Explaining the Mechanics of a Bird's Nest," Physics, vol. 15, no. 72 (May 13, 2022).

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