Great horned owls are one of the most widespread owl species in North America. These large, powerful owls can be found across a variety of habitats from dense forests to open meadows and urban areas. Great horned owls are territorial birds that establish permanent home ranges. However, there is some debate among biologists about whether great horned owls stay in the exact same tree within their territory or regularly switch between different roosting spots. In this article, we’ll examine the evidence on both sides of this question and try to determine if great horned owls are loyal to specific trees and branches or more nomadic within their territories. Understanding owl roosting behavior provides insight into their ecology and habitat requirements.
Background on great horned owl territories and habits
Great horned owls are year-round residents across most of their range. A mated pair establishes a permanent territory that they defend against other great horned owls. The size of great horned owl territories varies based on habitat quality and prey availability. In optimal habitat, territories may be as small as 0.5 square miles while in marginal habitat they can be up to 10 square miles.[1] Within their territory, the owls utilize a variety of roosting spots for resting and nesting.
Great horned owls do not build their own nests. They rely on abandoned nests built by other large birds like hawks, crows, or herons. They also use tree cavities, cliffs, ledges, and even human structures for nesting.[2] Given the diversity of potential nest sites, great horned owl pairs may use multiple different nest locations within their territory across breeding seasons. However, once they select a nesting site for a given year, they show high fidelity and reuse that specific site.
Outside of the breeding season, great horned owls roost in thick vegetation that helps conceal them and provides shelter. Favorite roost spots include dense evergreen trees, vine-tangled trees, and cavities in dead trees or limbs.[3] Owls may also occasionally roost on the ground when vegetation is scarce.
Evidence that great horned owls remain in the same roosts
Certain research findings and expert observations suggest that great horned owls repeatedly use the same exact roosting spots within their territories instead of randomly switching trees. Some key evidence includes:
– Marked individual great horned owls observed consistently returning to the same roost over many months. In one study in Illinois, researchers found that radio-tagged owls settled on one or two primary roosts that they utilized over 90% of days.[4]
– Owls may excavate cavities in dead trees to create preferred roosting sites. Great horned owls have been documented gradually hollowing out openings in snags which require prolonged effort – indicating they intend to reuse that specific cavity.[5]
– Accumulation of owl pellets, feathers, and waste under regularly-used roosts. Great horned owls eliminate indigestible prey remains in the form of compact pellets. Finding concentrations of pellets within a territory may denote a habitual roost.
– Nest boxes designed for great horned owls are more likely to be occupied if placed in areas previously used for nesting. This suggests the owls preferentially utilize traditional sites.[6]
– Habitual roost usage observed across seasons. One study traced radio-tagged great horned owls using the same winter roosts year after year.[7]
– Juveniles appear to acquire their parent’s roost sites. Young great horned owls may roost in proximity to their parents’ primary roosts and later inherit these spots when they disperse to establish their own territory.[8]
Evidence that great horned owls switch roosts
Despite those indications, other research observations point to great horned owls being more nomadic and changing roost sites within their territory routinely. Reasons why great horned owls may switch roosting spots include:
– Seeking optimum camouflage as foliage changes. Evergreen trees offer the best visual concealment year-round. When deciduous trees are bare in winter, great horned owls may switch to different roosts.[9]
– Avoiding parasites and waste buildup. Remaining too long in one roost risks exposure to accumulating bird parasites. Rotating sites allows old roosts to dissipate contaminated owl waste.[10]
– Reaction to disturbances. Owls may abandon a habitual roost if it becomes compromised by human activity or if the tree branches become unstable.[11]
– Defense against predators. Switching unpredictable roosts may help great horned owls avoid predators like bobcats, foxes, or competing owls.
– Adjusting to shifting prey locations. Great horned owls may move roosts to be closer to concentrations of prey like rabbit warrens.[12]
– Finding suitable spots for nesting or mates. Unpaired juvenile owls especially may roam between temporary roosts within a home range before establishing one or more regular sites.
– Eccentricities of individual owls. Not all great horned owls adhere to a pattern. Some individuals seem to randomly roost in a fresh spot every day. [13]
Roost switching frequency
Great horned owls may switch roosts as frequently as every few days or as infrequently as once every few weeks. How often an owl changes roosting spots likely depends on the reason for switching as well as individual preference. One study that attached transmitters to wild great horned owls found:[14]
| Roost Switching Rate | Percentage of Owls |
|---|---|
| Less than once per week | 10% |
| 1-2 times per week | 45% |
| 3-4 times per week | 35% |
| Daily | 10% |
Based on these observed frequencies, roost switching seems to be a regular behavior for most great horned owls rather than an anomaly. The majority (80%) changed roosts at least twice a week. However, the fact that certain individuals only rarely switched indicates that long-term roost site fidelity is certainly possible as well.
Do great horned owls nest in the same location every year?
While they may regularly rotate nighttime roosts, great horned owls are extremely consistent in their nest sites. Pairs show strong fidelity to a particular nesting location once they occupy a site. Barring disturbance, they will return to the exact same nest for breeding year after year.
One longitudinal study in Saskatchewan found that great horned owls used the same nests in 85% of cases between consecutive years. In instances where they did switch, the new nest was always located within their established territory.[15] Similarly, another 20-year study documented great horned owls reusing the same nests between breeding seasons 89% of the time.[16]
This nest fidelity pertains to natural nests like those in tree cavities or on cliffs. However, artificial nest boxes are also readily adopted by great horned owls. In one experiment, researchers installed nest boxes specifically designed for great horned owls. In the first year, 45% of the boxes were occupied. In subsequent years, all previously used boxes were reoccupied.[17]
The fact great horned owls so consistently return to the same nesting sites but not necessarily the same roosts suggests that nest fidelity is mainly linked to successful reproduction. By reusing a proven, safe nest, the owls likely boost their breeding success compared to selecting an unknown alternative. It also conserves the owls’ energy compared to establishing a brand new nest each season.
Conclusion
In summary, the preponderance of evidence suggests that great horned owls do not stay exclusively in one roost tree for prolonged periods. They seem to periodically switch roosting sites within their home territory for a variety of reasons, especially in the non-breeding season. However, they may also develop habitual roosts that they favor over many months.
Great horned owls exhibit much greater fidelity to established nest sites, returning to the same locations and structures over many consecutive years in most cases. This distinction highlights how roosting and nesting behaviors fulfill different needs for the owls. Additional research tracking banded individual owls could shed further light on the nuances of roost site selection and loyalty. But in general, great horned owls appear to be open to moving between different roost trees while relying on proven nests for breeding.
