Western Screech-owl Conservation along the Shuswap River
Project Executive Summary
The macfarlanei subspecies of the western screech-owl (Megascops kennicottii macfarlanei) is a federally endangered owl that occurs in the dry southern interior of British Columbia. It is believed that fewer than 200 pairs occur in Canada. This species is a non-migratory resident that is assumed to rely on large, declining black cottonwood trees for nesting; loss of this habitat has been listed as the primary factor contributing to the current conservation concern. Effective conservation and habitat restoration efforts for screech-owls have been difficult to develop because very limited information is available about the ecology of this species in British Columbia, even though a conservation need has been clearly identified.
The purpose of this project was to collect information on the ecology of this species, including essential habitat requirements, so that effective population recovery can be attained. Research was needed to identify the link between screech-owls and riparian forests and determine which features of these forests are needed for nesting, foraging, and roosting. By following radio-tagged birds, we hoped to identify these features and determine the relative importance of each to life-cycle limiting factors affecting population viability. Secondly, the extension component was meant to engage landowners in active stewardship of important habitats and provide them with tools to conserve, enhance, and restore habitats to increase the productivity of screech-owls. The final component of the program assessed changes in behaviour and perceptions of landowners and feedback from end-users to increase program effectiveness.
We conducted call-playback surveys for western screech-owls at 286 stations situated throughout valley-bottom habitats to the east and north of Vernon, British Columbia from 2004 to 2008 to better define the distribution of the species in this area. We detected screech-owls 59 times at 45 stations, with most detections occurring along the Shuswap River between Cherryville and Shuswap Falls. We also detected owls along BX Creek and Coldstream Creek to the northeast and south of Vernon.
The distribution of screech-owls in our survey area was quite disjointed, which was likely related to the distribution of important riparian habitats for this species. Because of the disparate distribution of suitable habitat along valley-bottom areas, the most likely linkage from the Shuswap River population of screech-owls to the Okanagan populations may be through BX Creek. It is clear that targeted conservation programs are needed to help this population of owls persist.
We collected much useful information about screech-owls during call-playback surveys that will be useful for refining future survey efforts. These include: surveys should include broadcasting female calls during the last 2 weeks of March to determine whether pairs of owls are present, surveys sites should occur within (not adjacent to) suitable habitat, and survey locations should be close to trees to provide perching sites, among others.
To collect the ecological information about screech-owls that was vitally needed to help direct effective conservation, we captured, radio-tagged, and monitored 11 adult screech-owls (6M, 5F) between July 2005 and January 2008. We collected 704 radiolocations of these owls in 2688 radio-days of monitoring to evaluate home range sizes and spatial organization, habitat relationships, and population characteristics.
Screech-owls in our study had large home ranges that averaged 64.5 ha (SD = 10.6, n = 5), with no substantial difference in size between males and females. Owls used considerably smaller areas during the breeding season (x¯ = 20.4 ha, SD = 15.3, n = 7) than the non-breeding season (x¯ = 88.6 ha, SD = 44.5, n = 6). During the breeding season, males and females overlapped extensively, whereas outside the breeding season, males and females used different areas with very little overlap. We did not detect overlap of owls that were not part of a pair (i.e., no overlap with adjacent home ranges).
Riparian forests seemed to be a necessary component of home ranges of screech-owls; about 12 ha (or 18% of total home range area) of riparian forested habitats was needed for owls to occupy an area; we did not detect any screech-owls in areas that did not supply this critical density.
Screech-owls are a secondary cavity nester and a supply of suitable nest cavities are needed to support breeding. We identified 6 nests used by owls during 11 reproductive seasons; all nests were within cavities in large-diameter deciduous trees. Five nests were in large-diameter cottonwood trees (x¯ = 81 cm dbh, range 43-111 cm) and one was in a large paper birch (70 cm dbh). The cavities that the owls used for nesting were created through natural decay processes (branch hole cavities) and by primary cavity nesters and occurred an average of 14 m above ground. Trees that had these features were extremely uncommon; we estimated that <0.4% of the trees in our study area were remotely similar in size and decay class as those used for nesting. After females started incubating eggs, they were observed to leave the nest between 16 and 26 minutes after sunset (x¯ = 26 min., n = 5) and be away from the nest between 8 and 21 minutes (x¯ = 14 min., SD = 5, n = 5). Later in the nesting period, females were detected being away from the nest for up to 43 minutes at a time.
Screech-owls were very specific in the trees that they used for roosting, choosing trees largely based on their diameter. Owls were most likely to roost in trees that were between 48 and 90 cm dbh. However, when they used sites that did not have large trees, they selected patches of habitat that had considerable cover of trees and shrubs >2 m high and little cover below 2 m. We believe that owls selected trees and patches of habitat that provided either cryptic (camouflage) cover (i.e., large diameter trees that were the same colour as the owls) or concealment cover (i.e., dense vegetation) that hid them from potential predators or harassment from songbirds. At least 16% of roost trees were used more than once, with 2 different trees being used 7 times each.
Habitats with the following features can be considered essential for western screech-owls:
1) Roosting: trees with diameters between 48 and 90 cm dbh or patches of habitat with high densities of trees >40 cm dbh, considerable tree and high-shrub cover, and little low-shrub cover.
2) Nesting: cottonwood and paper birch trees that form cavities of sufficient size (i.e., internal cavity ≥19 cm wide).
3) Home Range Occupancy: on average 12 ha of riparian forest habitat within a 65-ha area that includes a mixture of zonal and open forests and early structural stage habitats (e.g., grassland or pasture).
We collected much useful information on the population processes of screech-owls along the Shuswap River. The rate of successful nesting in our research area (11 of 13 nest-seasons; 85%) was similar to that observed in other areas and the number of fledglings per nest (3.25) was higher than that reported for southern California. However, mortality and turnover in the breeding territories was also high. Three of 10 radio-tagged owls died: 2 owls (1 M, 1F) from different territories were killed by predators (likely great horned or barred owls) and 1 female owl was struck and killed on a secondary road. We observed 9 instances of territory turnover in 13 opportunities (69%). The average minimum life span of owls in our study was 1.92 years (SD = 0.72, n = 8), with the longest minimum life span of 3.3 years. Given these parameter estimates, it is unclear whether the population of screech-owls along the Shuswap River is stable.
Many of the research and inventory results will help with the recovery of western screech-owl populations in British Columbia. The conservation implications of our work include:
1) Conservation efforts can be focussed within the refined distribution of screech-owls in the Shuswap and northeastern Okanagan regions.
2) Improvements to survey methods will enhance the probability of detecting resident screech-owls.
3) Information on space-use and habitat associations can be used to improve the use of survey data in the estimation of density and population size.
4) Empirical information on the size and location of home ranges can be used to identify other areas that may support screech-owls.
5) Changes in space-use by screech-owls throughout the year can be used to identify areas outside of riparian zones that should receive targeted conservation efforts.
6) Nest cavities and the processes that create them appear to be life-cycle limiting factors for screech-owls. Land managers can use this information to conserve or restore habitats that support these rare habitat features.
7) Screech-owls have very specific requirements for roosting, which appear to be met in a narrow range of habitat conditions. Roost sites must provide cover, either in the form of cryptic (camouflage) or concealment cover.
8) Data-driven predictive habitat models have been developed that can be used for assessing habitat value, predicting changes in habitat value under various management scenarios, and help with the conservation of high-value habitats in other areas.
9) The diet of western screech-owls has been identified. Land management that favours the retention of foraging habitats should be promoted.
10) Essential habitat delineations will help regulatory agencies and forest licensees to refine Section 7 schedules and notices for screech-owls.
11) Effective Wildlife Habitat Areas can be better delineated based upon an improved understanding of the space-use and habitat requirements of screech-owls.
12) The linkage between screech-owls and riparian forests with deciduous components has been strongly characterized. Understanding the reasons that screech-owls need these habitats will promote land management activities that help conserve and restore these identified habitats.
Extension of Project Findings
We delivered an extensive outreach program to landowners along the Shuswap River. Between 2004-2008, we provided over 80 landowners with information on our project. In 2008, we distributed 43 stewardship manuals to people who owned land within home ranges of radio-tracked owls. Landowners were very supportive of the project; 97% of landowners allowed access to their land.
To assess the effectiveness of this extension product and the outreach program as a whole, an independent extension specialist conducted an evaluation of the program. Overall, feedback was excellent. Everyone who was interviewed found the information easy to understand and all of them felt that it was a very worthwhile project in which to have participated. Many of the landowners were already motivated to protect important habitat on their land, but almost all of them felt like they had more knowledge because of the information they had received from the manual. Many landowners were particularly interested in the information about rare species that live in their area, and were excited to talk about the ones that they had seen.
Through our extension efforts, we have facilitated two conservation covenants for significant screech-owl habitats along the Shuswap River. The Land Conservancy has agreed to hold a covenant for one property (approximately 7 ha) that was extensively used by 2 pairs of radio-tagged screech-owls. We have also been working towards a conservation covenant for a much larger section of land owned by BC Hydro that includes over 90 ha of the only old-growth cottonwood riparian forest (essential habitat for screech-owls) in the project area.
Extensive outreach was conducted throughout the project. Presentations on preliminary results of the project were given to 3 Okanagan naturalist clubs, the BC Field Ornithologists annual meeting and at the Federation of BC Naturalists AGM. One scientific paper on the diet of western screech-owls has already been accepted for publication and 3 others are being prepared for submission. Results from the project have also been used to design Wildlife Habitat Areas in BC and in a BC status report on the macfarlanei subspecies of screech-owl.
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