Effects of Experimental Removal of Barred Owls on Population Demography of Northern Spotted Owls in the Pacific Northwest

Science Center Objects

Barred owls (Strix varia) have expanded their geographic range from eastern to western North America, and their newly expanded range now completely overlaps that of the federally threatened northern spotted owl (S. occidentalis caurina; Gutiérrez et al. 2007, Livezey 2009, USFWS 2013). Evidence from long-term demographic studies indicates that the presence of barred owls has contributed significantly to declines in spotted owl populations over the past two decades (Anthony et al. 2010, Forsman et al. 2011, Dugger et al. 2015). Interspecific competition with an increasing number of barred owls that establish and defend their territories in habitats occupied by spotted owls may explain why populations of spotted owls have continued to decline even though considerable amounts of forested habitat has been conserved throughout the subspecies’ range. Previous research by USGS (Wiens et al. 2014) showed that the presence of barred owls can alter the behavior and fitness potential of spotted owls, as shown by changes in movements, habitat use, and reproductive output of spotted owls exposed to varied levels of spatial overlap with recently established barred owls.

Project Description

barred owl sitting on a tree branch

Barred owl (Strix varia). Photo by Mark Musselman, National Audubon Society.

Large-scale removal experiments have been identified as being uniquely capable of identifying the explicit role of competition with barred owls in population declines of spotted owls (Gutiérrez et al. 2007, Buchanan et al. 2007, Forsman et al. 2011). Experimental removal of barred owls has also been identified as a high priority recovery action for the northern spotted owl (USFWS 2011). In response, the U.S. Fish and Wildlife Service (USFWS) released a Final Environmental Impact Statement (EIS) and Record of Decision for the experimental removal of barred owls to benefit threatened northern spotted owls (USFWS 2013). The documents reiterate earlier perspectives on the need to implement large-scale, controlled experiments to determine how barred owls are implicated in population declines of spotted owls (Gutiérrez et al. 2007, Buchanan et al. 2007, Johnson et al. 2008), and provide a formalized framework for implementing experiments to help guide future management decisions.

This study will determine whether experimental removal of barred owls has a positive influence on populations of spotted owls. The study has been designed to specifically address the overarching research question: Does removal of barred owls ultimately improve population trends of northern spotted owls? The experiment will provide a direct test of whether competitive interactions with barred owls translate to demographic consequences for spotted owls, and if so, whether active management of barred owls can improve population trends of spotted owls.

Objectives

Northern spotted owl grabbing a mouse off a branch

Northern spotted owls (Strix occidentalis caurina). Photo by Emily Brouwer, Mount Rainier National Park

The goal of the study is to test the research hypothesis that the population rate of change of spotted owls, or one of the demographic components driving population change (survival, reproduction, recruitment, site occupancy dynamics), is negatively influenced by the presence of barred owls. The removal study will be conducted over at least four years to allow sufficient time for populations of both species to respond to experimental treatments (USFWS 2013). Specific objectives of this study are to:

  1. Determine the effect of experimental removal of barred owls on population dynamics of territorial spotted owls with respect to site-occupancy dynamics, reproductive output, survival, recruitment, and annual rate of population change.
  2. Estimate pre- and post-removal differences in the occurrence and colonization of barred owls in control versus treatment areas.
  3. Estimate the amount of effort and cost required to maintain sufficiently low numbers of barred owls and achieve positive effects on populations of spotted owls.

Experimental Study Areas

The study will be spatially replicated on up to four study areas distributed over the range of the northern spotted owl, including one in Washington (Cle Elum), two in Oregon (Coast Ranges, Union/Myrtle/Klamath), and one in northern California (Hoopa/Willow Creek). These areas were selected based on many considerations, including availability of pre-treatment demographic information on spotted owls, land ownership, and the need to identify the effect of barred owls on spotted owls across the broad range of forest conditions occupied by both owl species (USFWS 2013).

With the exception of the Union/Myrtle treatment areas in southwestern Oregon, all experimental study areas are part of a long-term demographic monitoring program for northern spotted owls established under the Northwest Forest Plan (Lint et al. 1999, Davis et al. 2011). All study areas, including Union/Myrtle, contain greater than 20 years of mark-recapture data on spotted owls and between 55 and 198 historical spotted owl sites each (total number of historical spotted owl sites = 471). The study areas are comprised of mostly federal lands, but some field work may occur on adjacent private lands with the permission of the landowner because of the mixed composition of land ownership in some areas. Including a mixture of ownership is important in order to understand the effects of experimental barred owl removal across the wide range of barred owl densities and forest habitat conditions that occur within the range of the spotted owl. The study areas vary in climate, vegetation composition, and topography, but all are dominated by conifer or mixed conifer-hardwood forests (Dugger et al. 2015).

Methods

Predicted Outcomes and Experimental Design Considerations

Based on previous studies of population demography and competitive interactions between spotted owls and barred owls, we posit that if interactions between the two species translate to fitness costs to spotted owls, then the predicted outcomes of competitive release (i.e., reduction in numbers of territorial barred owls) include:

The study uses a before-after, control-impact experimental design, where barred owls will be removed from treatment areas that have been monitored for both species prior to the treatments, to compare with data after barred owl removals begin to see if there is a positive response in populations of spotted owls in the treatment areas. To designate treatment and control areas, we identified continuous blocks in each area with similar conditions with respect to: 1) number of historical spotted owl territories needed in each control and treatment area (n=50) to reliably detect changes in population trends and site-occupancy dynamics (Johnson et al. 2008, USFWS 2013), and 2) forest structural conditions within owl sites, based on previous research suggesting that the presence of barred owls may interact with forest structural characteristics around nesting areas to affect site-occupancy dynamics of spotted owls (Dugger et al. 2011, Sovern et al. 2014).

Owl Surveys and Demographic Monitoring

Species-specific surveys of spotted owls and barred owls will be conducted on control and treatment portions of each study area during the breeding season (1 Mar – 31 Aug). For spotted owls, surveys and demographic monitoring will continue to be conducted by biologists and agencies already responsible for the long-term demographic monitoring of northern spotted owls under the Northwest Forest Plan (Lint et al. 1999, Davis et al. 2011). Survey and monitoring of spotted owls will be based on historically occupied breeding territories that have been consistently monitored since 1990 so that results from the experiment are consistent with previous analyses of population demography and occupancy dynamics of spotted owls ((Anthony et al. 2006, Forsman et al. 2011, Dugger et al. 2015).

Barred owls will be surveyed on treatment and control portions of all study areas to locate barred owls for removals and document pre- and post-treatment changes in the site-specific probability of occupancy, extinction, and colonization. Our study will use a survey protocol specifically developed for barred owls (USFWS 2013) in combination with the general occupancy survey design developed by Wiens et al. (2011) to track annual changes in site occupancy dynamics and areas used by barred owls on control and treatment areas.

Barred Owl Removal Protocols

Following USFWS (2013) and Diller et al. (2014), this study will primarily use lethal removal methods for barred owls, but non-lethal removals will also be used in cases where previous arrangements have been made to relocate live captures. Lethal removal of barred owls detected in treatment areas during breeding season surveys will be accomplished using a 12- or 20-guage shotgun with non-toxic bird shot. For non-lethal removals, barred owls will be captured with baited Dho-gaza nets, baited mist nets, snare poles, noose poles, pan traps, or by hand (Clark, 1981; Forsman, 1983). Our protocol for removals prohibits collection of nesting barred owls with dependent young (USFWS 2013). As a consequence, removals will generally occur during the nonbreeding season (Sep – Mar), especially during fall and winter months after young have gained independence from adults and dispersed from their natal areas. We anticipate frequent colonization of barred owls into areas where barred owls have been removed (Yackulic et al. 2014, Diller et al. 2014). Once a pair of barred owls is removed from a site, we will attempt to conduct regular follow-up visits to determine re-colonization rates of treatment areas by barred owls following removals. Appropriate federal and state permits for removals were obtained prior to any removal activities. All carcasses of barred owls removed from experimental treatment areas will be collected, measured, and preserved as museum specimens for many future scientific studies.

Literature Cited

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