A control program was implemented in British Columbia in 2007 to support recovery of the endangered Northern Spotted Owl (Strix occidentalis caurina) by removing Barred Owls (Strix varia), competitors that are contributing to the Northern Spotted Owl?s population decline. From 2007 to 2016, a 10-year period, 150 Barred Owls were removed from 32 sites that are managed for Northern Spotted Owls. Barred Owls were removed mostly by live capture and translocation to outside the Northern Spotted Owl range (soft removal, n = 108), but lethal removal methods (n = 42) were used if the Barred Owl imminently threatened a Northern Spotted Owl. Soft removal required 1.5 times more person-days per removal than lethal methods. Consistent inventory and removal efforts were undertaken at 19 of the 32 removal sites, and those sites were used to examine Barred Owl site-level response. The number of detected Barred Owls per site was significantly higher (n = 19, t = −6.42, P < 0.01) pre-removal (mean 5.7, SD 2.14) compared to the 2016 post-removal year (mean 2.5, SD 2.2), and an overall annual mean of 0.83 (SD 0.95) Barred Owls were removed per site per year. Barred Owls were absent from only two sites in the 2016 inventory year. Barred Owls that were thought to be new colonizers were detected at sites annually, as confirmed by the positive overall mean annual rate of return of Barred Owls per site per year (n = 19, R ? = 0.38). Occupancy modelling indicated the best-fit model, assuming a fixed detection probability of p = 1.0, resulted in constant and high occupancy (ψ > 0.82) due to positive recolonization (γ = 0.726) and low turnover despite previous removals (ε = 0.164 removals compared to ε = 0.159 no previous removals). Northern Spotted Owl response to Barred Owl removals was difficult to determine due to the low numbers of Northern Spotted Owls and the methods used to confirm their status at sites. Overall, the study?s findings were limited by small samples and the operational nature of data collection, which warrants caution in overinterpreting the results for management actions. The findings of this study, however, combined with findings from experimental Barred Owl control studies in the United States, can be used to assist the next phases of the British Columbia Barred Owl control program.
Joel Gillis, F. Louise Waterhouse. 2020. Barred Owl Removal Report 2007-2016. FLNRORD. TR128