How Do Bird Nests Stay Together? Researchers Unravel Entanglement Between Stiff, Straight Rods

taylorvich writes:

https://phys.org/news/2025-04-bird-stay-unravel-entanglement-stiff.html

The concept of constructing a self-supporting structure made of rods-without the use of nails, ropes, or glue-dates back to Leonardo da Vinci. In the Codex Atlanticus, da Vinci illustrated a design for a self-supporting bridge across a river, which can be easily demonstrated using toothpicks, matches, or chopsticks. However, this design is fragile-pulling one of the rods or pushing the bridge from below can cause it to collapse.

In contrast, bird nests-which are also self-supporting structures consisting of rigid sticks and twigs-are remarkably stable despite continuous disturbances such as wind, ground vibrations, and the landing or takeoff of birds. What makes bird nests so sturdy?

This was the question at the center of a recent paper from L. Mahadevan and his team at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). The research is published in the Proceedings of the National Academy of Sciences.

Mahadevan is the Lola England de Valpine Professor of Applied Mathematics, of Organismic and Evolutionary Biology, and of Physics at SEAS and the Faculty of Arts and Sciences at Harvard. The paper was co-authored by Thomas Plumb-Reyes and Hao-Yu Greg Lin.

While entanglement in small, flexible systems, such as polymers, is well understood, less is known about how stiff, macroscale components entangle, especially when they are densely packed.

"When we think about entanglement, we typically think about flexible, individual constituents wrapping around each other, as exemplified in tangled headphone cords or entangling vines," said Mahadevan. "Contrary to this common intuition, stiff and straight rods can also entangle themselves-if they are long or thin enough."

To understand how, the researchers used X-ray tomography-a technique that creates a detailed cross-section of an object-as well as computer simulation and experimentation to peer inside and reconstruct the complex structure of bird nests.

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