To build a nest, a bird does not want to take any old twigs. Somehow the birds choose a material that will create a cozy, strong nest.

“It’s completely mystifying for me,” said Hunter King, a physicist at Akron University in Ohio. The birds seem to have a sense of how the properties of a single rod will translate into the characteristics of the nest. This connection “is something we don’t know for the first time about forecasting,” King said.

A bird’s nest is a special version of granular material: a substance like sand made up of much smaller objects (SN: 30.04.19). King and colleagues combined laboratory experiments and computer simulations to better understand oddities of granular nest-like materialsreported the researchers in a study in which they appear Physical examination letters.

In the experiments, the piston repeatedly compressed 460 bamboo rods scattered inside the cylinder. Computer simulations allow researchers to analyze the points where the rods touch, which is key to understanding the material, the team said.

The greater the force applied by the piston to the pile, the harder the pile becomes, which means that it can withstand further deformation. When drilling the piston, the rods slide against each other and the contact points between them are rearranged. This hardened the pile, allowing additional contact points to form between the rods, which prevented them from bending further, the simulations showed.

Changes in the stiffness of the pile seem to have lagged behind the movement of the piston, a phenomenon called hysteresis. This effect made the pile harder when the piston was pushed in than when the material bounced back when the piston was retracted. The simulations suggest that the hysteresis occurred because the initial friction between the rods had to be overcome before the contact points began to rearrange.

Beyond bird’s nests, this research can be applied to other materials made from unarranged stacks of long fibers, such as felt. With a better understanding of the physical properties of such materials, engineers could use them to create new structures designed to protect not only bird eggs but also other loads that people consider valuable.

Experiments hint at why bird nests are so sturdy

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