When you think about raptors, maybe you remember a fun science exercise from your past where you dissected owl pellets. Dissecting pellets is a fun and useful way to gain information on diet! Studying what animals eat provides a way to examine species interactions in addition to gaining important insight into ecological relationships.
Most raptors produce pellets consisting of indigestible remains of their prey, including bones, teeth, scales, hair, feathers, keratin, and chitin, which makes up the exoskeletons of insects. Other carnivorous birds, such as herons, kingfishers, and gulls, also produce regurgitated pellets. Pellets can look similar to mammalian carnivore droppings, such as coyotes, but generally lack the twisting along the long axis. Mammalian scat also usually has a distinct smell.
For our study focused on Golden Eagle predation of Greater Sage-Grouse, we collected and analyzed pellets to determine the overall prevalence of Sage-Grouse in the diet of Golden Eagles near Vernal, Utah. We also incorporated DNA metabarcoding, a technique that allows you to identify DNA fragments of many different species from a single sample, such as feces. Combined with Golden Eagle nest cameras, we incorporated three different methods to determine whether sage-grouse was present in the diet.
You may be wondering why we are curious about whether Golden Eagles are consuming Sage-Grouse. Greater Sage-Grouse are declining throughout most of their range in the West. Much of the decline has been attributed to the loss of quality sagebrush habitat. Although Golden Eagles mainly prey on rabbits and rodents, they are often considered one of the most formidable predators of adult Greater Sage-Grouse. In our study area, land managers observed Golden Eagles perching on power poles near Sage-Gouse leks. Leks are breeding locations where males and females congregate each spring for courtship displays and mating. To gain a better understanding of Golden Eagle and Sage-Grouse interactions, we paired our diet study with behavioral observations of Sage-Grouse responding to various predators during the lek.
Identifying what bird a pellet came from can be difficult, as pellets can vary greatly and overlap in size with those of other species. While nests can be great places to find pellets, we limited disturbance to nests by collecting pellets only when accessing Golden Eagle nests to band chicks and deploy GPS transmitters. Instead, we collected the majority of pellet samples from Golden Eagles perching on power poles. To ensure success, we often waited long periods in the field for eagles to produce samples or leave their perches.
Ultimately, we collected and dissected 35 pellets! Dissections were a long and tedious process completed mostly by Utah State University undergraduate, Riian Brown, over the course of a semester. We found all sorts of interesting remains, including teeth, nails, scales, and many bone fragments. Due to the experience and museum collection specimens needed to match all the remains, we focused on identifying the feather samples to achieve our research goals, which focused on the prevalence of Sage-Grouse in the diet. We found feathers in 26% of samples. Most of the feathers consisted of small covert feathers and were not identifiable. These small body feathers are not as easy to identify as remiges and rectrices (wing and tail feathers). Luckily, we were able to identify the species present in four samples by matching them with study specimens in the lab. We found Greater Sage-Grouse in three samples and a Northern Flicker in one sample. The pellets with Sage-Grouse feathers also contained Sage-Grouse feet! All pellets contained bone material, 89% contained hair from mammals, and 11% contained scales! Overall, mammals represented the bulk of the diet in our pellet samples.
Consistent with our DNA metabarcoding and nest camera methods, our pellet samples revealed that Sage-Grouse was not a primary or important item in Golden Eagle diets. We didn’t record any Sage-Grouse delivered to our three nest cameras and only found Sage-Grouse in 15% of feces samples that contained both Golden Eagle and prey species DNA. Although we found evidence of Golden Eagle predation of Sage-Grouse through two of our three methods studying diet, it doesn’t appear that Sage-Grouse are an important source of prey for Golden Eagles. Sage-Grouse evolved alongside many predators, and it is unlikely that Golden Eagles are playing a significant role in their continued decline.
If you want to try your dissection skills at home, you can order an owl pellet from the HawkWatch International store: https://hawkwatch-international.myshopify.com/products/owl-pellet
This blog was written by HWI’s Field Biologist, Tori Thorpe. You can learn more about Tori here.
