The only thing I could hear as I glided through the peaceful woodland on an early April morning was the hiss of my skis on crusty snow. I paused short and searched the woods ahead with binoculars I had tucked under my jacket for warmth. I had a feeling the goshawk nest tree was nearby. After a few more steps, my gaze was drawn to a black nest mass 150 feet ahead, high in a tree. I pushed my poles into the snow and strained in the weak light to see whether a grey head or tail tip protruded over the nest rim.
Two piercing yellow eyes set in a round grey visage burst into view as my gaze travelled slowly over the tangle of sticks 40 feet above the ground, looking down at me. Suddenly, the Great Gray Owl turned its head, as if to eliminate me from both its gaze and its universe. For a brief minute, I believed I was hallucinating. The bird just vanished, its feathers merging in with the nest. However, the golden eyes resurfaced. I secretly rejoiced as I lowered my binoculars and withdrew back into the woods.
Such an incident has been quite typical since 1989, when I first began monitoring the Northern Goshawk population on the Targhee National Forest in western Wyoming and eastern Idaho. But the thrill of watching forest raptor nests hasn’t worn off, nor has my joy in realising that researching goshawks in the Northern Rockies will afford so many chances to see Great Gray Owls.
The Northern Goshawk’s range in western North America ranges from the boreal woods of Alaska and Canada south to Arizona and New Mexico. The goshawk is the biggest of North America’s three Accipiter (true hawk) species. The goshawk, as well as its two smaller relatives, the Sharp-shinned Hawk and the Cooper’s Hawk, all breed in the Targhee National Forest and have structural adaptations for life in wooded habitats. They can fly through thick forest stands with incredible speed and agility because to their wide, short wings and long, rudder-like tails. To ambush or flush out unsuspecting prey from cover, all three species utilise stealth and cunning.
The goshawk is well-known among falconers and bird watchers for its aggressive foraging habits, which include catching rabbits, snowshoe hares, tree squirrels, woodpeckers, and forest grouse. The goshawk is far more difficult to spot and watch than the similarly sized Red-tailed Hawk, which hunts from high perches or flies over broad territory. Often, all that can be seen is a short grey streak vanishing into a dense forest.
Prior to my research, nothing was known about the goshawk’s nesting habitat or life cycle in the northern Rockies. On the Targhee National Forest’s 1.8 million acres, records comprised of a few historic nest sites and sporadic observations of individual birds. I was hired to examine potential nesting sites and gather data on nesting ecology and environment.
More information was available on the Great Gray Owl, which Alan Franklin had examined on the Targhee National Forest in the early 1980s. This owl’s distribution in the western United States is more limited than that of the goshawk. It stretches from Alaska and Canada’s boreal forests south via the Sierras and Cascades to central California, and from the Rockies to western Wyoming. The lodgepole pine woods in the Island Park region, just west of Yellowstone National Park, were the focus of Franklin’s research. The owls were supposed to have congregated there due to large populations of pocket gophers in the area’s many clear cuts. In response to a mountain pine beetle infestation, the Forest Service launched an intense salvage harvest campaign in the early 1970s.
The Great Gray Owl, like other owls, does not build its own nests but instead utilises broken-top snags, mistletoe platforms, or old hawk nests. According to Franklin, 60% of the owls he observed nested on broken-top snags. His findings did not reveal that the great grey utilised goshawk nests extensively, nor that they were related with the Douglas fir woods that constitute up 50% of the Targhee Forest’s wooded environment.
Because of their link with older-age forests, interest in forest raptors has grown significantly since Franklin’s research. Both the Northern Goshawk and the Great Gray Owl have been recognised as Sensitive Species in the Targhee, as well as many other western national forests. Wildlife species whose populations may be threatened by management operations such as wood harvesting or fire control are examples of sensitive species. Furthermore, the goshawk has been recognised as a Management Indicator Species (MIS) for mature and old growth forest throughout much of the western United States. As part of its mission to preserve the ecological health of mature forest habitat on our national forest lands, the Forest Service is mandated to monitor goshawk nesting numbers.
Monitoring hawk populations is mainly based on how often hawks utilise the same nests or nesting grounds. If hawks return to the same nesting spot year after year (for example, Peregrine Falcons on specific cliff ledges), rigorous yearly assessments of all known nest sites will provide an accurate evaluation of overall reproductive success. To evaluate population changes in hawks that utilise various nest locations each year, substantially more intensive survey work is necessary. Prior to my surveys, I reviewed goshawk literature from different places and discovered inconsistent findings on how often goshawks reuse the same nest tree. Some studies found that goshawks often returned to the same nest tree year after year, while others found that goshawks rotated between two to three different trees placed up to a quarter mile apart.
Based on these data, I estimated that it would take three to four years of intense surveys to find the bulk of current goshawk nest trees in recognised nesting regions. I believed that after I discovered the majority of the nest trees, population monitoring would become much simpler, needing yearly spring trips to known nest stands to calculate the number of occupied goshawk territories each year. However, like with most natural occurrences, the system revealed to be more complicated and dynamic than it originally looked.
During the first three years of nest monitoring (1990-1992), I discovered new alternative nest trees in most territories and a bigger number of nesting goshawks each year, as projected. In 1992, the most successful breeding year, I discovered 21 inhabited nests. However, during the following several years, the number of occupied territories fell to less than half.
After examining 31 breeding areas for six years, it is clear that both the number of territories inhabited by goshawks and the quantity of young they produce each year vary substantially. Much of this fluctuation seems to be weather-related. Goshawks do badly in years with cold, rainy springs, with many nests producing just one, if any, offspring. Warm, dry springs result in great productivity: nests may contain two or three young, or even four on exceptional occasions.
I also discovered that the total number of alternative nest trees per territory and the position of nests in respect to one another vary greatly. The number of nest trees per territory varies from one to eight, with an average of three. The average distance between alternative nest trees utilised in successive years exceeds 1000 feet, although lengths might exceed 3/4 mile. Because of these traits, determining whether or not goshawks are nesting in a certain region in a given year is very difficult and time intensive.
The more alternative nest trees located in an area, the more probable it was that goshawks would return to nest in later years, according to one pattern that developed. The best nesting environment seems to be related with a large number of alternative nest trees. But why do goshawks create so many distinct nests?
Building a nest is not a simple process; it requires a significant investment of both energy and time, especially considering the difficult task that awaits a nesting couple. From late March, when courting starts, until the nestlings hatch in mid-June, the male goshawk must provide or feed the female. If he does not supply adequate food during the early nesting season, the female may either not lay eggs or will abandon the nest to forage for herself, exposing the eggs to potentially lethal chilly temperatures or predators. The time spent by the male on nest construction rather than food collection might be the difference between success and failure in rearing a brood.
Birds establish secondary nests for a variety of reasons, including predators, parasites, and pressure from nest rivals, according to researchers. In most circumstances, it is difficult to determine which element is to blame. Because of the lengthy, very cold winters, I questioned if nest parasites were a significant impact. Other nearby raptors big and powerful enough to fight for nests with the goshawk, such as the Great Horned Owl, Red-tailed Hawk, and Swainson’s Hawk, normally nest in more open area, reducing the chance of nest rivalry unless in heavily disturbed forest habitat. According to my observations of nests, goshawks were also quite capable of guarding their nests from other aerial predators that occurred to wander nearby.
The Great Gray Owl was the only big raptor that could be found in deep, mature woods on the Targhee. However, despite their intimidating appearance, they weigh far less than the Great Horned Owl. Through prey analysis, I also discovered that there is very little diet overlap between the goshawk and the great grey owl, since the owl’s diet consists of the nocturnal, fossorial pocket gopher. Based on what I understood about the two species, it seemed that the goshawk would pose a greater danger to the Great Gray Owl than the Great Gray Owl, and that nest competition from Great Gray Owls was improbable. My results, on the other hand, point to a more complicated relationship between the two species.
I recorded 89 goshawk nest trees between 1989 and 1995 and rechecked 76 of them between 1 and 5 years. Over a six-year period, I discovered nesting raptors 47 times out of 227 total nest checks: 18 Northern Goshawks, 26 Great Gray Owls, and one each Great Horned Owl, Long-eared Owl, and Cooper’s Hawk.
Not only did I locate more nesting Great Gray Owls than goshawks, but the owls utilised the same nest tree more often than the hawks. I only found goshawks nesting in the same tree in successive years twice. Great Gray Owls, on the other hand, nested four years in a row at one nest and three years in a row at another.
Even more astonishing, I discovered great greys and goshawks breeding in the same woodland stand. This was documented 13 times in eight different countries. The distance between active owl and hawk nests varied from 364 to 2890 feet, with an average distance of 1300 feet. The owls always successfully fledged their offspring under these settings (that is, young survived at least until they could leave the nest). Goshawk nests failed twice, both during the early portion of the nesting season. These unsuccessful nests were 466 and 925 feet from active owl nests, respectively.
Great Gray Owls seem to use goshawk territory as a source of dependable nest constructions. Two incidents that I was fortunate enough to see appeared to show that the owls could compete with the more aggressive goshawk. First, I discovered Great Gray Owls nesting in an older goshawk stick nest that had clearly been constructed by goshawks earlier that year. I also discovered goshawks nesting in the neighbourhood, but in a recently made nest. It has been recorded in Northern Europe that great greys may take over goshawk nests after the hawks had begun to repair them during the pre-nesting season.
Second, I saw an adult male Great Gray Owl feeding three fledglings near an active goshawk nest with young in it. The young owls were screaming out for food from the top of a huge exposed Douglas fir snag. While I sat and observed, the male owl delivered three pocket gophers to feed his starving offspring within 15 minutes of each other. Meanwhile, the nesting goshawks became aware of my presence and sat or flew over me three or four times, vocalising loudly. The owls and goshawks, which were only about 200 feet apart, could hear and see each other. Even though the juvenile owls seemed to be easy pickings for the goshawks, the owls did not change their behaviour or attempt to defend themselves. The owls seemed to be entirely ignored by the goshawks.
This final occurrence suggests that, in addition to utilising goshawk nests, owls may have another benefit. During the day, owls prefer to rest and are less active and vigilant than hawks. The noisy defensive action of goshawks might suffice to alert even the most slumbering owl to the possibility of danger. Perhaps this also works in reverse, with the owls protecting the nest stand at night while the goshawk sits closely on the nest. Interestingly, both species finish egg-laying around the same time (mid-April to early May), a synchrony that might be impacted by competitive or mutualistic ties between these two species. Even while both of these species are capable of killing each other’s young and presumably do so at times, particularly when the young are only partly developed and left undefended, they may accept and support each other in ways we have yet to learn under specific conditions.
The most significant lesson for land managers to take up from this goshawk-owl nest sharing is that controlling owl nesting habitat also involves managing goshawk nesting habitat. The goshawk’s significance as an indicator species seems to be warranted in this circumstance. Finding and keeping forest stands including all alternative goshawk nests, as well as managing enough foraging habitat, is critical for long-term conservation of both species and their prey in wood harvest zones.
So, if you chance to be trekking between May and July in Montana, Idaho, Wyoming, or eastern Washington and Oregon, where the ranges of these two species overlap, keep an eye out for these forest phantoms. You could be lucky enough to come upon an example of territorial time-sharing.
Alan B. Franklin, 1988, Great Gray Owl breeding biology in southern Idaho and northern Wyoming. Condor 90, pp. 689-696/
Hawks, Eagles, and Falcons of North America, Paul A. Johnsgard, 1990. Smithsonian Institution Press, Washington, D.C.
North American Owls, Paul A. Johnsgard, Smithsonian Press, Washington, D.C., 1988.
Robert W. Nero, The Great Gray Owl, Smithsonian Press, Washington, DC, 1980.
Susan M. Patla. 1997. Northern Goshawk nesting ecology and habitat in undisturbed and wood harvest zones of the Targhee National Forest, Greater Yellowstone Ecosystem. Idaho State University, MS Thesis ID Pocatello
Susan Patla has a Master of Science degree on the goshawk from Idaho State University. She is the nongame biologist for Wyoming Game and Fish Department in Jackson, WY. And as a research associate for Northern Rockies Conservation Cooperative, she continues to monitor goshawks on the Targhee NF.