Western Screech-owl Conservation along the Shuswap
River
Project Executive Summary
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Final Report View Stewardship Manual
View Screech-owl Fact Sheet
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.
Inventory
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.
Research
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.
Conservation Implications
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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