I live in a rural setting, about 35 minutes from downtown Seattle, in an environment of mixed forest, individual homes, and farmland. My neighbor has a pair of nesting Northern Barred Owls (NBO) living on or near their property and they have been photographing the birds for several years now. They recently shared their photo collection with me and that sparked my interest in NBOs, which lead to researching and writing this story.
It has been said that the NBO and our Spotted Owl (SO) don’t play well together, and a reason this is so was spelled out so eloquently by GF Gause in his classic work on Paramecium (1935, The struggle for existence) where he proposed what later became known as the “competitive exclusion principle,” one of ecology’s few guiding principles: Two species cannot permanently coexist unless they are doing things differently.
The SO has lived on North America’s west coast for a very long time, while the NBO has arrived in
relatively recent times. The black and white map is from a 1976 paper. The Northern Barred Owl (NBO) range is shown as the darker section and the Northern Spotted Owl (NSO) range is the lighter, dotted section. Note the absence of range overlap in this 1976 report. Several sightings of the NBO, including nests and dead birds, were found in NSO territory by mid-1970, but apparently there were not enough of them to convince the authors of permanent NBO residency on NSO turf. The prevailing opinion at the time was that the NBO had not expanded into NSO territory by the mid-1970s.
We can see just how rapidly the incursion into the central-to-north west coast of North America took place by looking at the next image {colored) taken from a 2021 research paper by Wiens et. al. In about 45 years the NBO moved from Eastern British Columbia to the Pacific Ocean, up to Alaska, and down to California, completely engulfing the range
of the SO. The NBOs were reported in southwestern Montana by 1909, in northern British
Columbia by 1943, and in southern BC by 1959. The first reported sighting of an NBO in Washington State was in 1965, in Idaho in 1968. A sighting was reported in 1972 in Oregon, and some were reported in Northern California by 1976, so the expansion of the NBO throughout the continent’s central west coast is indeed rapid and recent.
The NBO is native to eastern North America and until about 11,500 years ago the NBO could not possibly venture outside its home territory. NBOs require a mature forest for nesting and foraging, and Canada was buried under at least a mile-thick ice sheet. The peak thickness occurred about 22,000 years ago, so it would have been impossible for an NBO to survive up north until the ice had melted and the land had reforested. East-West boundaries for NBO habitat at that time, as well as today, were the Atlantic Ocean and the Great Plains, neither of which had forests, and still don’t. Current thinking favors westward NBO expansion starting less than a century ago (no reported sightings out west before then) with a pathway going north along the Missouri River valley into Canada where there were trees, then west.
The NBO-NSO Link
By the mid-1980s alarms were being sounded by activists decrying the observed decline in NSO numbers, and they attributed the decline to habitat loss through old-growth logging, a “common sense” culprit. Lawsuits were filed and in 1990 the US Fish and Wildlife Service listed the NSO as threatened. The Northwest Forest Plan, imposing limitations on timber harvest in NSO territory, was adopted in 1994 in hopes that the NSO population decline would cease. Unfortunately, the decline continued.
Habitat loss, at some level, must be an important driver for NSO population reduction, but apparently not the only causative variable. From 1994 through 2007, various authors identified competition between the NSO and the newly arrived NBO as a potentially causative factor. For the first time the NBO was being thought of as an enemy species, one that was seriously out-competing the NSO for survival.
How Can the Northern Barred Owl Out-Compete the Spotted Owl?
What appears to be happening is that the NBO population is replacing the NSO population and there are at least three strong reasons (food, reproduction, and aggressiveness) to expect this state of affairs other than SOs being more sensitive to habitat loss, which will drive them to extinction even if NBOs did not exist. As a side note, the NBO requires forests, but not necessarily old growth ones, so they can do well where the NSO cannot..
1. Food. A recent paper by Valeria Briones reviewed literature on NBO and NSO diets. While details vary with forest types and season of the year, the general upshot is that the NSO feeds mainly on differing proportions of Northern Flying Squirrel, deer mice, wood rats, voles, moles and hares with very little else included. Their prey is dominated by nocturnal mammals, so nearly all of the NSO hunting is nocturnal. The NBO includes all of the above in its diet, but also preys on diurnal species such as invertebrates, a few fish, and birds, some of which can be hunted in daytime in more open terrain. For example, the NBO has been seen in Nova Scotia foraging in forest debris for earthworms, while they have been seen in Washington State eating slugs. Thus, the NBO is dietarily better suited for survival than the NSO due to its ability to secure additional food sources from the same terrain.
NBOs do eat other birds. The pictures show the nest (circled in red) of a Barn Swallow under the eaves of my neighbor’s
house, a nest an NBO has also observed. The other photo shows the NBO with its prey taken from the nest, one that’s about to be swallowed head-first, as is their method of eating. The NBO took all nestlings,one at a time, and was seen (but not photographed) giving one to another NBO.
2. Reproduction. Owls pair up at age two and start nesting at age three. They stay together for life (~20 years) and take good care of their owlets, who leave the nest a month after hatching, using talons, beaks, and wings to maneuver around the tree hosting their nest. Owlets stay with their parents for another two months until they reach full adult size and can fly, leaving soon thereafter. NBO owlets stay with their parents a bit longer than do NSO owlets.
A 2014 paper by JD Wiens et. al. describes a study that followed 29 radio-marked NSOs and 28 NBOs for two years in a 745 square-km study area. An interesting finding was that “… pairs of barred owls produced an average of 4.4 times more owlets than pairs of spotted owls over a 3-year period”. Moreover, the NSO produced fewer owlets the closer their nest was to one of an NBO pair. Also, none of the NSO pairs nesting within 1.5 km of a nesting pair of NBOs produced owlets. Finally, a breeding pair of NBOs usually produce owlets every year, while NSO pairs may skip a year or more.
3. Aggressiveness. Every paper I read discussing NBOs and NSOs together remarked that NBOs are the more aggressive owl, but none provided evidence or references. Perhaps it’s common knowledge that an amateur such as myself would be unaware of. Anecdotally, my neighbor reports being dive-bombed more than once by the NBOs she was photographing. She has never seen an NSO and cannot comment on their attitude toward her. The above paper (Wiens, 2014) seems to indicate NSOs avoid contact with NBOs when possible. At any rate the next paper attempted to quantify aggressiveness, yet the results appeared to me to be somewhat equivocal.
Van Lanen et. al. report results of an experiment designed to quantify aggressive behavior between potentially competing owls. Taxidermy mock-ups of NBOs and NSOs were made, calls from each owl were prerecorded, the mock-ups were placed in either NBO or NSO territory, and a response call (if any) was recorded as being from an NBO or an NSO. Whenever the responding owl would attack the mock-up owl, the strike force was measured. Results were not clear and convincing, since 15 logistic models were fit and R-squared was between 0.18 and 0.38. Curiously, the most frequent attack was by an NSO on an NSO mock-up. Still, the authors claimed minimal support for the aggressiveness hypothesis.
What to do about it — if anything?
The Northwest Forest Plan of 1994 slowed old growth logging, but it didn’t slow down the declining NSO population, likely because it either didn’t go far enough or didn’t address the NBO encroachment. In the early 2000s some were suggesting the NSO could be salvaged if NBOs were removed from the NSO range. A small pilot program was initiated and the results suggested there might be a positive outcome for the NSO.
Hence a multi-site, multi-year study (2009-2019) was designed and carried out. The authors state “…After removals, the estimated mean annual rate of population change for spotted owls stabilized in areas with removals (0.2% decline per year), but continued to decline sharply in areas without removals (12.1% decline per year).
I noticed that the reports on “removal” of NBOs didn’t say how the removal was to be accomplished. Further investigation uncovered the removal mechanism: 12-gauge shotguns. Nearly 2,400 NBOs were killed for this study.
By now it should be clear that the NBO seems well-positioned to survive at the expense of the NSO, which may not survive extinctions grasp. Note the title of this article from The Guardian, April 16, 2021, “How Canada is trying to protect its last three spotted owls”. There are a few other owls in breeding sanctuaries, where it is hoped they can provide a source of new owls. Among other features, a one year halt to all logging in old growth forests was to be enforced. Within a year The Narwhal reports the British Columbia government had initiated plans to open up formerly pristine old growth to logging — again.