Barred owls (Strix varia) are increasingly being viewed as a significant threat to the northern spotted owl (Strix occidentalis caurina), a species that has been declining across its range for decades. Understanding the nature of this threat and how best to address it is an important concern in spotted owl conservation efforts.
Quick Introduction to Barred and Spotted Owls
Barred owls and spotted owls are both medium-sized owl species that nest in tree cavities or broken treetops. Historically, they occupied separate geographical ranges, with spotted owls largely confined to old growth forests west of the Rocky Mountains and barred owls found in forests east of the Rockies. However, in the past century barred owls have substantially expanded their range westward and now broadly overlap with spotted owls across the Pacific Northwest and parts of western Canada.
Barred Owl Range Expansion
Several factors may have facilitated the barred owl’s westward expansion:
- An increase in wooded corridors along waterways, roads, and railways that allowed barred owls to gradually spread west
- Adaptability to a wider range of forest types and fragmentation than spotted owls
- Lack of competitive exclusion from spotted owls due to initial spatial separation
Spotted Owl Declines
Meanwhile, spotted owl populations have declined 3-4% per year across significant parts of their historic range. Major causes include:
- Habitat loss from timber harvesting and land clearing
- Forest fragmentation from roads, development, and other causes
- Competition from barred owls (an emerging and growing threat)
The spotted owl was listed as Threatened under the U.S. Endangered Species Act in 1990, largely driven by declines from habitat loss and fragmentation. Barred owl competition was not yet viewed as a major concern.
How Barred Owls Compete with Spotted Owls
There are two main ways barred owls outcompete spotted owls:
1. Aggressive displacement and even mortality
Barred owls tend to be more aggressive than spotted owls. They will physically attack spotted owls and chase them from their territories. In some encounters, barred owls have even killed spotted owls.
2. Resource and habitat competition
Barred owls are able to outcompete spotted owls for critical resources like nesting sites, roosting sites, and food. They also utilize a broader range of forest types and can survive in more fragmented landscapes. This gives them an advantage over spotted owls which need intact old growth forest.
Evidence of Impacts on Spotted Owls
There is mounting research demonstrating how barred owl presence negatively impacts spotted owl site occupancy, reproduction, and survival:
- Spotted owls are less likely to occupy historic territory in the presence of barred owls.
- Even when not displaced, spotted owl pairs produce fewer young when barred owls are nearby.
- Spotted owl population trends decline more rapidly in sites where barred owls are well established.
- Survival rates of spotted owls decrease in areas where they overlap with barred owls.
One study covering 11 years of data found that spotted owl survival was significantly lower in barred owl occupied areas vs. unoccupied areas. The table below summarizes the results:
| Study Area | Barred Owl Presence | Annual Survival Rate of Spotted Owls |
|---|---|---|
| Washington | Absent | 0.818 |
| Washington | Present | 0.645 |
| Oregon | Absent | 0.800 |
| Oregon | Present | 0.646 |
This type of empirical data shows the clear detrimental effects of barred owls on spotted owl survival in areas where they co-occur.
Barred Owl Removal Experiments
Given the mounting evidence that barred owl presence causes significant harm to spotted owls, some management agencies have conducted experimental removals of barred owls from spotted owl territories. The intent is to see if reducing barred owl numbers within localized areas benefits spotted owl occupancy and reproduction.
One study in northern California found that removing barred owls from historic spotted owl territories led to increased spotted owl detections. Whereas spotted owl occupancy was only 15% in occupied barred owl removal sites pre-removal, occupancy increased to 73% in the two years post-removal.
Other experiments in Oregon and Washington have yielded mixed results, with some spotted owl pairs benefiting from barred owl removal but others showing little population response. More research is still needed, but selectively removing barred owls shows promise for improving spotted owl numbers in certain contexts.
Criticisms of Barred Owl Removal
Barred owl removals are controversial. Some key concerns include:
- Expense and feasibility of removing enough barred owls to make a difference
- Difficulty in sustaining removals, as barred owls re-colonize vacant territories
- Harm to individual barred owls, as protected native birds
- Questionable ethics of killing one species to benefit another
Despite these criticisms, the U.S. Fish and Wildlife Service has expanded removal experiments, viewing the threat posed by barred owls as severe enough to warrant such actions in selected areas important to spotted owl recovery.
Other Conservation Actions
In addition to localized barred owl removals, experts recommend these other conservation measures to benefit spotted owls:
- Increased habitat protections on federal and state lands
- Restricting old growth forest harvesting on private timberlands
- Developing spotted owl habitat corridors between fragmented public land patches
- Continuing research on innovative conservation strategies
A combined approach is likely needed, but barred owl removals are likely to remain a controversial component going forward.
Conclusion
In summary, barred owls pose a significant threat to already declining spotted owl populations throughout their western range. Barred owls outcompete spotted owls for habitat and resources and also directly interfere with and displace spotted owls. Studies clearly demonstrate reduced spotted owl site occupancy, reproduction, and survival rates in areas where barred owls are present. Targeted barred owl removal shows promise for improving spotted owl numbers in localized areas, but remains controversial. A multifaceted conservation strategy is recommended, but the barred owl threat will likely grow without direct management intervention.
