Skeleton remains from a Barred Owl pellet found on Bainbridge Island | Olivia Cavalluzzi
by Olivia Cavalluzzi
While other students spent their summer interning at marketing agencies or working retail positions, my summer job was collecting coyote scat. I worked at a University of Washington lab that studied the diet and genetic distributions of urban coyotes using their scat, and I spent 8 hours a day hiking trails, abandoned fields, and parks to collect all the coyote scat I could find.
I was on Bainbridge Island, taking my dog out on my latest scat-finding adventure, when we stumbled across something we were not expecting. Around the base of three tightly packed trees was a sea of white, a bleached bed of tiny bones. I had seen my fair share of owl pellets, but they were always few and far between and always more fur than bone. This was a different sight entirely; an unbelievable number of bones were scattered in a dense blanket across this small patch of trees. My dog backed away, unsure about this rodent graveyard, but I knew I could not walk away from our new discovery.
After buying hundreds of small bags, I returned that afternoon. And the next day. And the day after that. At the end of each day, I was pulling more skulls and femurs out of the tangles of my hair than I cared to count. I kept this up until the forest floor was as unassuming as the surrounding areas and the backseat of my car was filled with 345 bags of owl pellets.
Bags of skeletons from pellets found on Bainbridge Island | Olivia Cavalluzzi
For owl lovers, Washington State is the place to be. From the Olympic Peninsula to the dry eastern border, 15 species of owls have been reported in Washington. Using the feathers mixed in with the bones, I was able to identify that the pellets I found were from a Barred Owl. With vertical barring on its chest and a distinctive “Who cooks for you? Who cooks for you all?” call, the Barred Owl (Strix varia) is one of the most common owls in the Seattle area. Barred Owls are native to eastern parts of the United States, but their range has drastically expanded over the past century to the West Coast due to human interference. When white settlers colonized the United States, they suppressed wildfires and built shelterbelts along the Great Plains. This removed the natural barrier in forest cover that had previously limited the range of Barred Owls and facilitated their rapid expansion westward. This dramatic range shift, in addition to habitat loss and climate change, has led to the endangerment of the Northern Spotted Owl (Strix occidentalis caurina), as it is being outcompeted by the newly prevalent East Coast arrival. As Barred Owl numbers grow and Northern Spotted Owl numbers reach dangerous lows, research efforts into both species are becoming increasingly important. Two years into a research paper on different avian scavengers, a car filled with Barred Owl pellets was the perfect place for me to continue studying raptors and contribute to Barred Owl research.
Barred Owl in an urban forest | Olivia Cavalluzzi
Barred Owl pellet with fur, feathers, bones | Olivia Cavalluzzi
From their near-silent flight to turning their heads 270 degrees without breaking vessels or tendons, there is a lot to love about owls. What continues to fascinate me the most, however, is how they digest their prey. Like most raptors, owls swallow their prey whole or in large segments. However, they cannot digest all parts of most animals. When an owl first swallows its prey, it goes head first down their beak, through their esophagus, and into their glandular stomach, or proventriculus, where all of the digestible parts are liquified. The meal then passes on to the muscular stomach, or gizzard, which blocks the indigestible matter and compacts it into an oval of hair, scales, bones, teeth, feathers, keratin, and chitin that they cough back up. While many raptors and other birds produce pellets, owls have weaker digestive juices and tear up their prey less than other birds, making owl pellets by far the most common. These regurgitated pellets provide a unique opportunity to noninvasively study the diet of an owl by dissecting the different elements contained inside them. The bones I had found around those trees were from hundreds of Barred Owl pellets, the fur degraded off the bones from three months of summer heat.
Once I had the tiny bones I had to figure out what animals they were from. Teeth and jaws are the best way to narrow bones down to genus, such as determining if they were from a rat, mouse, vole, mole, shrew, or something else entirely. Once I knew the genus, I used specimens from the Burke Museum to identify the specific species. By this point, I had used community science platforms and word of mouth to locate three other Barred Owl hotspots and had 573 owl pellets to process. With the help of volunteers, friends, and labmates, we identified the skulls and teeth in each pellet, resulting in 1,840 individual prey items belonging to 28 different species.
Pellet skeletons for identification | Olivia Cavalluzzi
I am still in the early stages of data analysis and have a ways to go before publishing my findings, but there is already a lot we can learn just from the raw data. Although pellets cannot determine the absence or abundance of a species, they can determine presence. My findings provide compelling evidence for which prey species are present in Seattle and where they are located. My findings can also suggest diet preference in Barred Owls. Townsend Voles (Microtus townsendii) were by far the most common, numbering 614 individuals. Despite the prevalence of squirrels, particularly Eastern Grey Squirrels (Sciurus carolinensis) in the greater Seattle area, I did not find any evidence of them in the pellets I studied. The remains of Eastern Cottontails (Sylvilagus floridanus), an increasingly common rabbit in Seattle, were only in 5 pellets from a single location.
With increasing urbanization, owl pellets also offer the opportunity for us to begin examining the ways in which owls can consume rodenticide, commonly known as rat poison. Mice and rats, the primary targets of rodenticide, made up 28% of the bones I collected, coming only second to voles. However, just because mice and rats are targeted does not mean they are the only ones being poisoned; nontarget small mammals can ingest the poison as well. Not only does this exposure pose a substantial risk for owls ingesting poisoned rodents, but it also poses a risk to our entire food web. Barred Owls are not the only ones eating small mammals, and rodenticide poses a risk to many other species. When rodenticides are present in the food web, the toxins can accumulate in higher-level predators as they continue to ingest poisoned prey. As Barred Owls consume toxic prey, they can then pass those toxins on to their own predators, such as Great Horned Owls.
Bait box containing rodenticide, used to control rodent populations
Barred Owl sharing rodent prey with its young | Jason Collins | Audubon Photography Awards
If you want to see an owl, Seattle is a great place to start. The best place to look for owls is in forested areas. Although there are often many sightings in Seattle’s large parks like Seward or Discovery, you may get lucky finding one in a tree in your own neighborhood. Dawn and dusk are ideal owl finding times , especially if you want to see them flying or hear them calling. Using community science apps like eBird or iNaturalist can increase your chances to locate owl hotspots near you. Looking for large streaks and patches of white owl excrement will help to narrow down their favorite perch and hunting trees and almost always helps me find pellets. When you are out looking for owls, make sure to check around your feet at the base of trees as well, as you might just find some owl pellets of your own to dissect.
About Olivia Cavalluzzi
Olivia is a wildlife biologist, multimedia artist, and environmental educator who lives in Bellevue, WA with her partner, Rebecca, and their kittens, Finn and Rowan. She has also worked at Birds Connect Seattle as a Nature Camp Naturalist, sharing her love of birds and nature with children and teens. Visit ColorOfCreativeShop on Etsy to purchase sustainable earrings made from the bones in her owl pellet studies!
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