1. What Owl Pellets Are and How They Record an Owl's Hidden Meals
Owls swallow small prey whole. A part of the stomach called the gizzard separates digestible parts from those that cannot be digested. Only bones, teeth, and fur are gathered into an oval-shaped lump, which is spat out through the mouth a few hours later. This is called a pellet, and it is different from droppings [1].
For birds like eagles or hawks that tear their prey apart, bones become crushed and hard to identify. Because owls swallow whole, the bones remain close to their original shape, providing clues about what was eaten recently. By looking at skulls, researchers say they can narrow down whether the meal was a rodent, reptile, amphibian, or bird [1].
No one sees the meal itself, but the record falls to the ground. So, what discoveries are actually made when we decode this record?
2. What 67 Long-Eared Owl Pellets in Ehime Prefecture Revealed About Food Sources
A study examined 67 pellets from long-eared owls that overwintered in a village in Ehime Prefecture. This report by Kawaguchi and Yamamoto of the Ehime Prefectural Museum of Nature and History was published in the Ornithological Science journal in 2003 [2].
The researchers found bones from rodents like house mice, bats, shrews, and one whole bird. Among these, four species—Japanese grass mouse, Japanese pipistrelle, whiskered bat, and lesser white-toothed shrew—were recorded as food for long-eared owls for the first time [2].
It is important to note that these are not "new species" of animals. The animals themselves were already known. Rather, this was a new record confirming for the first time that long-eared owls eat them. Owl diets are filled in one entry at a time like this. So, when we want to know which animals live in an area, can owl pellets replace surveys that use traps?
3. Do Owl Pellets or Traps Find More Small Mammal Species?
The traditional method for studying small mammals is setting up traps to catch them. Studies comparing pellets and traps do not always yield the same results.
In a study in Spain, the number of species found was 17 from owl pellets, 14 from genets (a nocturnal carnivore similar to cats), and 9 from live traps. Dividing 17 by 9 gives about 1.9, meaning pellets picked up nearly twice as many species [5].
However, along the Araguaia River in central Brazil, researchers examined 286 barn owl pellets (151 from one nest site and 135 from another). They found two rodent species that appeared only in the pellets. On the other hand, four species caught only by traps did not appear in the pellets, so the number of species was higher with traps. The authors concluded that the two methods complement each other [4].
One reason may be that owls have biases in where they hunt and which prey they choose. Traps are biased toward the place where they are set, while pellets are biased toward the area around the nest. So, what happens when we look at change over a long period of time?
4. How Pellets in Cave Layers and Museum Collections Show Long-Term Change
In Homestead Cave in Utah, USA, owl pellets accumulated in layers over thousands of years. By examining the bones in each layer, researchers can identify which small mammals lived nearby during that time.
According to a commentary on a 2015 PNAS study by Terry and Rowe, when temperatures suddenly warmed about 13,000 years ago, the species of small mammals changed, but the overall "energy flow" of the community remained stable. However, since the late 1800s, human land changes (such as the spread of invasive cheatgrass) are said to have significantly reduced biomass [6].
This is a result from this specific cave and cannot be directly extended to other places. Still, being able to extract records spanning thousands of years from one cave is possible because pellets accumulated in many layers.
At the Zoology Museum of the University of Navarra in Spain, more than 72,000 specimens, including skulls and lower jaws derived from barn owl pellets, are stored. The records cover more than 40 years starting from 1967, and they are said to be increasing through a monitoring network that began in 2015 [7]. Records of different time lengths, from thousands of years to decades, remain as bones.
5. How DNA from Pellet Bones Hints at Unnamed Rodents
Sometimes it is difficult to distinguish between similar animals based on bone shape alone. Therefore, a study examined the DNA (genetic information) from bones in barn owl pellets in central Brazil (Systematics and Biodiversity, 2015).
Usable sequences were obtained from about three-quarters of the samples, identifying 11 mammal species. Two opossum species were recorded for the first time at the study site. Two rodent species (genus Oecomys) did not match any known species and are said to possibly be undescribed species [8].
This is not a confirmed stage of "new species discovery." However, it showed that DNA can provide clues for identifying animals that are hard to distinguish by bone shape alone. So far, we have looked at what pellets are good at. Where are the points where mistakes are likely?
6. Why Counting Bones in Pellets Does Not Directly Measure an Owl's Diet
There are two points to be careful about when reading pellets.
The first is how easily bones remain. Small prey are swallowed whole, so bones and teeth tend to remain. Large prey are torn apart, so bones are less likely to remain. Therefore, the number of bones does not necessarily equal the proportion of food eaten [3]. If roosts or nests are found, pellets are relatively easy to collect, but one must consider that they may be biased toward foods whose bones remain easily.
The second is the effect of digestion. A 2025 study (Greenwood et al., Ecology and Evolution) examined two owls kept in a zoo: one great horned owl and one red-tailed hawk, fed frozen rats. They found that the ratio of bone isotopes changed from that of the eaten rats due to digestion. It is said that caution is needed when using bones to estimate diet or environment [9].
This does not mean owl pellet bones cannot be used. It means we must distinguish between what can be read and what is difficult to read.
7. How to Safely Observe an Owl Pellet Yourself
If you find a lump of bones and fur in forests or shrines where owls live, do not touch it with bare hands; observe it with an adult. Wash your hands well if you touch it.
Some local museums and nature observation groups hold events where they break pellets apart to examine the bones. Check if such an event is being held nearby.
If you want to read materials, a Japanese summary of the Ehime report [2] is available. Try comparing the names of the four species found with the text.