a handy tool for casting the enclosed nest structure of birds – Methods Blog

Post provided by Jing-Chia Guo.

To understand something, we often describe its appearance and shape: The ball is round, the can is cylinder, and the pillow is kind of rectangle. However, most natural creatures are irregular in shape, so it’s difficult for people to quantify or define them. Sometimes, scientists are even unable to get their hands on the objects they need, and that is a real pain for me too. I was excited to have a chance to study Asian house martin (Delichon dasypus)—a songbird species that builds its nests under man-made buildings. One of my goals was to understand how their nest structure influences nest microclimate. Well, it sounds easy, but really gave my brain a hard workout. I will share the story behind our recently published methodology in MEE.

We started from a quick idea, and then things became complicated

Back in 2021, I was discussing my thesis topics with my advisors. We all knew that one main function of a nest is to maintain suitable microclimate for offspring, as shown in many studies on birds and other animals. So, we thought that focusing on this topic would be a safe bet for the thesis project even if other parts did not go well. But we were wrong. First, the Asian house martins’ nests are made of mud and are enclosed with a single narrow entrance (Fig. 1a). These characters make it difficult to measure their internal structure and almost impossible to take them down without breaking them. Second, traditional structure measurements—which assume the shapes of objects as classic geometric types such as squares, circles or ovals—are not suitable for Asian house martins’ nests with irregular shapes. So, our “safe bet” turned out to be not secure after all.

I wish that the “pups” in Prometheus were real and able to automatically map the structures by lidar. But, as far as I know, they only exist in the movie! I started wondering if there is any material that is flexible, can harden, and even expand outward as it fills space so that I can use it to cast martin nests? The answer came to me one day when shopping at a hardware store near my house—polyurethane! It is a liquid-liked foaming agent commonly used in construction to fill gaps between materials. I tried to injected polyurethane into the nest and produced an excellent replica of the nest’s interior (Fig. 1b; see video here). Yet, I was not the first one to use polyurethane for casting enclosed structures. Forty years ago, it was used to map the pocket gophers’ burrows for understanding their structure. It was also applied to create deer mouse burrow casts in a more recent study. Clearly, it is a useful tool for capturing animal burrow structures. However, I failed to find detailed information from the literature on how to apply polyurethane to other subjects. Thus, I had to figure out a practicable approach to obtain the replica of Asian house martin nests’ interior structure using polyurethane.

Nest character measurements in both manual and digital ways

Measuring nest characteristics on the replicas made things easier. We adopted two classic methods, Archimedes’ drainage method and drawing combined with imageJ program, to measure the nests’ interior volume and entrance size, respectively (Fig. 2). While manual measurements worked well for these characteristics, other complex features were harder to assess from the polyurethane replicas. Hence, we started to think: what if we made a 3D model for it? By doing so, we were able to obtain data with higher quality and better potentials.

Let’s skip the detailed procedure of 3D modelling covered in the paper. The 3D model looked spot-on (Fig. 3), but we still worried about potential biases in modelling. To double-check, we measured both interior volume and entrance size again but using a software called Metashape (see video here), and compared them to the manual measurements. Thrillingly, they are almost identical! That means that 3D modelling is a powerful tool to accurately recreate real-world objects.

Science in everyday life, and some tips of our approach

The thing I learned from this process is that science is closer to us than we thought. You can use everyday tools from grocery or hardware stores to solve scientific challenges and share your discoveries to others. Our method can be used to capture the internal space of various animal-made structures, and helps you to evaluate their shape or other characters. You can use polyurethane on its own or combined with 3D modelling, depending on your purposes. Before you dive in, here are some tips:

1. Other materials, like resin, have similar functions. Pick the one that fits your requirement best.
2. Polyurethane is sticky! Make sure to protect anything it may contact, including your hands and clothes.
3. Polyurethane expands and can sometimes damage the object you are casting, so practice beforehand!

Enjoying creating your casts!

Post edited by Lydia Morley