Effects of experimental removal of barred owls on population demography of northern spotted owls in Washington and Oregon—2016 progress report

Abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS (1-888-275-8747).For an overview of USGS information products, including maps, imagery, and publications, visit https:/store.usgs.gov.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: Wiens, J.D., Dugger, K.M., Lewicki, K.E., and Simon, D.C., 2017, Effects of experimental removal of barred owls on population demography of northern spotted owls in Washington and Oregon-2016 progress report: U.S. Geological Survey Open-File Report 2017-1040 Figure 6. Change in the proportion of barred owls collected that were either adults (3 years or older) or subadults (1-2 years old) during the first 16 months after removals were initiated on three study areas, Washington and Oregon, 2015-16 ..................................................................................................................... Conversion Factors AbstractEvidence indicates that competition with invasive barred owls (Strix varia) is causing rapid declines in populations of northern spotted owls (S. occidentalis caurina), and that the long-term persistence of spotted owls may be in question without additional management intervention. A pilot study in California showed that removal of barred owls in combination with habitat conservation may be able to slow or even reverse population declines of spotted owls at local scales, but it remains unknown whether similar results can be obtained in areas with different forest conditions and a greater density of barred owls. In 2015, we implemented a before-after-control-impact (BACI) experimental design on three study areas in Oregon and Washington with at least 20 years of pre-treatment demographic data on spotted owls to determine if removal of barred owls can improve localized population trends of spotted owls. Here, we report on research accomplishments and preliminary results from the first 21 months (March 2015-December 2016) of the planned 5-year experiment.

Can culling Barred Owls save a declining Northern Spotted Owl population?

“…Removal experiments were first enacted on a small scale in California's Green Diamond study area in Humboldt and Del Norte counties in 2009 (Diller et al, 2016), in California's Hoopa Valley Reservation starting in 2013, and in the coastal ranges of Oregon and central Washington starting in (U.S. Fish & Wildlife Service, 2016Wiens, Dugger, Lewicki, & Simon, 2016). Using occupancy modeling, Diller et al (2016) report that lethal removal of Barred Owls improved Spotted Owl recovery in the Green Diamond study area.…”

“…Using occupancy modeling, Diller et al (2016) report that lethal removal of Barred Owls improved Spotted Owl recovery in the Green Diamond study area. However, this effect is local and likely temporary, and when culling subsides the global survival of the Northern Spotted Owl again comes into question (Wiens et al, 2016).…”

Review of the effects of barred owls on spotted owls

“…The rapid expansion of barred owls across the Pacific Northwest and, more recently, through the Sierra Nevada has motivated conservationists, researchers, and managers to assemble the local and regional management plans necessary to mitigate the harmful effects of barred owls on spotted owls. With the start of an experimental 5‐year plan to lethally remove barred owls from demographic study areas (Diller et al , ; Wiens et al , Wiens et al, ), there is evidence that removals can increase population growth of spotted owls in at least some areas. As noted earlier, evidence also suggests that quality habitat may buffer spotted owls from the negative influences of barred owls (Dugger et al ) and that spotted owls may have slight differences in use of forests and terrain, such as denser understory and steeper terrains, to carve out refugia (Jenkins et al ).…”

Effects of experimental removal of Barred Owls on population Demography of Northern Spotted Owls in Washington and Oregon—2018 Progress Report

“…Barred Owls detected in treatment areas during surveys were subsequently removed using 12-gauge shotguns loaded with nontoxic shot. We observed frequent recolonization by Barred Owls (Wiens et al 2018), so we did regular follow-up visits to detect colonizing Barred Owls and conduct additional removals as needed. We identified sex of Barred Owls based on vocalizations and morphometric measurements (Wiens and others, 2014).…”