What’s physics got to do with it?
We still have a few more days until spring, but March 11 seemed as good a day as any to learn about the physics underlying bird nests, from Professor Corey O’Hern. With real life examples of bird nests from the Yale Peabody Museum, we learnt about some of the features that determine the resilience of a nest. The work that Corey described, supported significantly by the National Science Foundation, included collaborations with Richard Prum and Kristof Zyskowski.
Bird nests are primarily a place for laying and incubating eggs, for raising the hatchlings, and protecting them from predators. Nests come in a variety of shapes and sizes, and can be built from diverse materials such as twigs, leaves, mud, grass, feathers, and sometimes even man-made materials, very often all held together with the bird’s saliva. The location of nests is also diverse and can range from trees and bushes to ledges.
An overview of the diversity in bird nest shape, size and location, courtesy L. Shyamal & Mahesh Iyer
Considering the extreme weather patterns that a bird nest is exposed to, the structural properties that confer the necessary resilience are worth exploring. These properties can be replicated to engineer materials for human-occupied structures like buildings and aircraft. The ability of a nest to withstand pressure is one factor being studied in Corey O’Hern’s laboratory, first by studying features in abandoned bird nests, and then developing computer models to refine our understanding of those features.
One measure of resilience or sturdiness of a material is its yield stress, or the (vertical) force at which it will be crushed. Accordingly, ceramics and metals have a high yield stress. Using footage from a recent PBS special featuring his work on bird nests, Corey showed us how he measures the yield stress of nests. What is remarkable is that the yield stress to break a cup nest, can be almost as high as that required to break ceramics; but the density of a nest is 10 times less than that of ceramics. So, while cup nests may sometimes appear to be breakable and haphazardly put together, they are exceptionally sturdy and the mass of the nest is roughly proportional to the mass of the parent.
Deconstructing the nest of a crissal thrasher by taking x-rays and CT scans; and then physically removing and measuring each piece that went into making the nest, computer models were developed to replicate the nest. The models are continually refined to improve their resilience to pressure. Factors such as the ratio of the length of a twig to its width (aspect ratio) are key to having a cohesive structure, and are included in the computer models, as well as the ability to withstand vibrations. Such studies can provide a quantitative understanding of the nest construction process, and the components that go into nest building. Refinements of these models can help in the development of strong lightweight materials for a variety of uses, as was described in a cafe on materials science led by Professor Sharvan Kumar.
Thank you, Corey, for showing us how strong a bird nest is, even if it appears terribly fragile. The video for the cafe will be available on Friday, March 17 at https://www.youtube.com/user/tildecafe
Here’s a hilarious take by Gary Larson, on the materials birds use to make their nests – this one would likely not be as resilient as the ones studied by Corey O’Hern and his colleagues!
To watch humming birds in their nests, visit http://cams.allaboutbirds.org/channel/52/Green-and-white_Hummingbird/. This video shows a robin reinforcing her nest with mud – https://youtu.be/1s2vpHka3PQ. For a weaver bird making its nest, check https://youtu.be/qbWM1QAVGzs