Particle physics experiments

Experiments suggest why bird’s nests are so strong

To build its nest, a bird will not look for just any old twig. One way or another, birds choose materials that will create a comfortable and strong nest.

“It’s just baffling to me,” says physicist Hunter King of the University of Akron in Ohio. Birds seem to have an idea of ​​how the properties of an individual stick will translate to the characteristics of the nest. This relationship “is something that we don’t know the first thing about the prediction,” King says.

A bird’s nest is a special version of a granular material: a substance, like sand, made up of many smaller objects (SN: 04/30/19). King and his colleagues combined laboratory experiments and computer simulations to better understand the quirks of nest-like granular materials, the researchers report in a study to be published in Physical examination letters.

In the experiments, a piston repeatedly compressed 460 scattered bamboo rods inside a cylinder. Computer simulations allow researchers to analyze stick contact points, which is key to understanding the material, the team says.

The more force the piston applied to the pile, the stiffer the pile became, which meant that it resisted further deformation. As the plunger sank, the sticks slid past each other and the points of contact between them rearranged. This strengthened the pile by allowing additional contact points to form between the sticks, which prevented them from flexing further, the simulations showed.

Changes in stack stiffness seemed to lag behind piston movement, a phenomenon called hysteresis. This effect made the pile stiffer when the piston pushed than when the material rebounded when the piston retracted. The simulations suggest that the hysteresis arose because the initial friction between the sticks had to be overcome before the contact points began to rearrange.

Beyond birds’ nests, this research could apply to other materials made up of disordered arrangements of long fibres, such as felt. With a better understanding of the physical qualities of these materials, engineers could use them to create new structures designed to protect not only bird eggs, but also other cargo that humans consider valuable.