Traditionally, the knowledge of naturalists has been based on observations. This approach is time-consuming because you have to spend hours in the field observing animals, plants, and natural processes. Also, sampling materials is an important part of gaining knowledge. However, in today's nature research, these methods are substituted by sampling with various devices such as audio recorders, trail cameras, or satellite monitoring. I understand that new methods bring a mass of data that is more precise or accurate than results from older ones. I have also used them very often. But... some old "classic" techniques are still useful. Maybe they are not appropriate for a study in a journal with a super high impact factor, but you can use them in serious research of specific parts of nature.
Natural history is best defined by the sampling and describing of natural materials. In zoology, searching for and picking up bones is one of these methods. I spend days each year looking for rocks or trees that represent owl roost sites. Owls regurgitate pellets containing bones from their prey. Today, this is used especially when you have installed nest boxes. Then it is easy to check them regularly. I search for bones under roost sites that are used outside of the breeding season. It is a combination of a survey (you have to find the roost site), experience (after some time you will know which sites they prefer), and knowledge (identification of bones is not easy work). When I started, I was lucky because my friend Michal Noga was one of a few experts on the determination of small mammal and bird bones. Thanks to his passion, I found an interesting field.
Before starting: why search for owl pellets or bones? A naturalist should see the value — bones are evidence of a species' presence at a site. Thanks to them, we can understand not only the current distribution of species but also their distribution and occurrence in the past. Owls are our assistants; they sample small mammals more effectively than zoologists. And the best part is — it is a non-invasive method.
Search for the roost site
The first step is to find a roost site. But where to start? You have to know details from the ecology of different owl species. Most of the time I search for Tawny Owls (Strix aluco), a medium-sized species with a very diverse diet. In forests, they like rocks and cliffs under tree crowns, entrances to caves, or old trees with cavities. If you find a roost site, it can often be used for decades. Especially roost sites in caves can be occupied since the Paleolithic. So, an owl's diet can tell us about the natural history of the location. It is fascinating!
Sampling of material
Hard work can start after the discovery of the site. If there are pellets, it is easy work; you can pick them up and store them in zip-lock plastic bags. In case you find bones on the ground, you should take samples of soil because bones can be under the surface. You will need a trowel and a bag for transporting samples. Your main tool for filtering bones from the soil and rocks will be a stream of water and a sieve. I use a robust wire mesh screen where I put a small volume of the soil sample and then put the sieve inside the stream. After a few seconds, you will see only rocks, wood, and bones.
Identification
This is the part that needs expertise. The best way is to compare bones with existing samples in museums or those owned by specialists in bone identification — osteologists. There exist some books appropriate for the identification of small mammals. I have used "Poznávame naše savce" by Miloš Anděra and Ivan Horáček. The last pages are dedicated to skulls and mandibles of mammals and their size information. Much information is available through scientific papers focused on individual species. Sources for the identification of mammals or amphibians are relatively available. However, there exist only a few books focused on the skulls of birds and bones such as the humerus and tarsometatarsus. Still, the best and most productive way is to prepare your own samples of bird bones.