The great gray owl is the largest owl in North America. Well, sort of. At about 27 inches, it is certainly the longest. Despite its large dimensions, though, it is comparatively lightweight, weighing nearly 25% less than the great horned owl and nearly 60% less than the snowy owl. As its name suggests, the great gray owl is gray overall, but that gray is admixed with black, white, and brown. It also has a conspicuously large facial disk with concentric rings of dark gray and a prominent white “bow-tie” below its yellow eyes and beak.
In California, the great gray owl occurs primarily in large, wet meadows in Yosemite National Park and surrounding forestlands. Although its typical habitat consists of large meadows between 4000 and 8300 feet elevation, it also occurs at lower elevations. Below 4000 feet, it inhabits drier meadows and oak woodlands where summertime temperatures often exceed 100 degrees Fahrenheit. The presence of great gray owls in such settings is truly remarkable though, as this species is adapted for cold climates and is famous for its ability to locate and capture prey under snow.
Great gray owls use a sit and wait hunting strategy. They perch on prominent branches, rocks, fence posts, and fallen logs, where they patiently watch and listen for unsuspecting rodent prey. Once a prey item is located, the owls make short flights, stall in mid-air immediately above their prey, and then plunge downward, extending their talons at the last second, enabling them to penetrate tall grasses, brush, gopher mounds, and yes, even snow. While California great gray owls will occasionally prey on lizards, birds, and small mammals such as flying squirrels, voles and pocket gophers comprise more than 90% of their diet.
Great grays nest in large diameter trees at the edges of large meadows. In California, great gray owls nest mainly in standing dead trees (snags) with broken tops. At higher elevations, they typically use red fir snags, while burls and broken limbs of black oak are typical nesting sites at lower elevations. Like most owls, great grays do not build a nest structure or modify the nest substrate.
Due to its limited population size (estimated to be less than 300 birds), isolated geographical distribution, and specialized habitat requirements, the great gray owl in California is state-listed as endangered. The Central Sierra Nevada of California represents the southernmost extent of the great gray’s global range. The core California population is centered at Yosemite National Park and the surrounding Stanislaus and Sierra National Forests. However, breeding records exist as far south as Sequoia National Forest and as far north as El Dorado National Forest. There is a gap of more than 200 miles in the range between the California population and the next known population to the north in southern Oregon. In 2007, a team of researchers led by the United States Forest Service set out to investigate whether this gap in distribution might indicate that great grays in California are genetically distinct from the owls to the north.
At the time, I was a recent college graduate hopping between raptor related field positions in Marin County and Plumas National Forest in California and Cape Charles in Virginia. When I heard of the opportunity to work with great grays in Yosemite, I was elated. I grew up in the foothills just south of Yosemite and have rich family ties to the park. Not to mention, I happen to think the great gray owl is one of the most fascinating birds on the planet!
As a member of the research team, I was involved in a variety of survey efforts for the owls, including trapping, banding, and collecting blood and feather samples for the genetics study. This work involved hiking into meadows at dusk and setting out traps baited with mice and affixed with radio transmitters. Many nights were cold and boring. We would sit in the dark all night shivering, waiting, and listening. But those nights that we did capture an owl completely made up for the tedium when we didn’t. Once captured, we measured the owl, affixed a uniquely coded leg band, and collected a small amount of blood and two small body feathers. The entire process took less than 20 minutes for each owl. Seeing the owls up close and personal was an amazing experience and made me appreciate even more just how spectacular these birds are. In hand, you really got a sense that the owls are mostly feathers—they are incredibly light for how large they are. In addition to the great grays, we incidentally captured other species of owls, including spotted, great horned, long-eared, northern-pygmy, and western screech.
In all, we collected genetic samples from about 35 great gray owls in California. These samples were processed in a lab at the University of California at Davis and compared with genetic samples from owls from Oregon, Idaho, and Canada. The genetic analysis showed that California great gray owls are genetically distinct from northern populations (Strix nebulosa nebulosa) and that subspecies status is warranted. The name proposed for this subspecies (Strix nebulosa yosemitensis) is a tribute to the national park where it thrives.
I continued working on the great gray owl project each breeding season (March to September) and eventually was presented with the opportunity to pursue a master’s degree at UC Davis as part of this great gray owl monitoring effort. Specifically, I set out to develop an alternative monitoring strategy using autonomous recording units (ARUs) to passively record and document great gray owls.
Traditional methods for surveying for great gray owls involve broadcasting recorded great gray owl vocalizations from a speaker at a set of standardized survey locations at each meadow of interest. The objective of this method is to elicit territorial responses from owls occupying the survey area. Although this technique is effective, it has several drawbacks. When working with an endangered population, care should be taken to minimize disturbances whenever possible. While eliciting a territorial response may be a minimal and temporary disturbance, ARUs have the advantage of providing a passive means of detecting the birds.
For my master’s degree, I set out ARUs at fifty meadows in the greater Yosemite area for two one-week intervals. I compiled a huge recording dataset that included over 40 terabytes of data in a single breeding season alone. Using sound analysis software designed by the Cornell Lab of Ornithology, I designed detectors to search for great gray owl vocalization events embedded in the recording dataset. Using this technique, I detected over 50,000 great gray owl vocalization events. I also developed models to estimate the probability that great grays would occupy a site and be detected. I then compared the results with those involving the use of the traditional broadcast technique. The ARU technique resulted in detection probabilities comparable to those afforded by the traditional technique and provided a cost effective, non-invasive alternative for survey and monitoring of great gray owls.
2014 marked the final field season for the US Forest Service led great gray owl project. However, we are still in the process of publishing our research findings. Specifically, I’m working on a predictive habitat model for great gray owls, a non-invasive genetic mark-recapture technique that relies on passively collected molted feathers, and expanding the work with the ARUs for aid in detecting nests and describing vocalization behaviors at active nests.
Great gray owls are rare, secretive, handsome, and elusive. For these reasons, they are one of the most sought after species in California by birdwatchers and park visitors. For those of you that want to try your luck at finding one in the Yosemite area, I have the following recommendations: arrive at the meadow about an hour before sunset, quietly walk along the edge (not through the middle) of the meadow until you find a good vantage point where an abundance of potential hunting perches exist (downed trees, rocks, small sapling trees, etc.), get comfortable, and quietly watch and listen for owls until the sun goes down. While it might take multiple attempts and requires some luck and patience, this is the best strategy to catch a glimpse of California’s “ghosts of the forest.”
– Joe Medley, Staff Scientist