What we know about screeches
My intention with this post is to provide a window into the developing state of knowledge for coastal Western Screech-Owl (Megascops kennicottii kennicottii) population trends and monitoring. Like any subject of scientific study, our understanding of this owl has developed over time as new and more complete information has come to light. The research objectives we are working towards with the Pacific Megascops Research Alliance (PMRA) are based on the state of knowledge that wildlife biologists in BC have developed over the past few decades regarding population trends and factors that might influence those trends. Of course, there’s plenty more known about coastal screech-owls beyond what’s presented here (check out Jeremiah’s bog blog where he explores and tests theories of habitat associations), but this should provide a primer for anyone interested in the motives and history behind BC Screech-Owl research and what PMRA is doing and proposing to do.
Hoping a rehabilitated screech has a place to go to (Photo: Samuelle Simard-Provencal)
Declines in Mind
Evidence of declining coastal screech-owl numbers first appeared in the 1990’s, primarily in the lower mainland and southeastern Vancouver Island BC. Local biologists and naturalists raised concerns about their population health, stemming from reduced detection rates and disappearance from an alarming number of previously occupied territories. In 2002, these concerns prompted the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) to list coastal screech-owls as Special Concern, followed by the same designation under the Species At Risk Act (SARA) in 2005. Continued declines were documented from subsequent monitoring, including more formal multi-year surveys, through the 2000’s leading to the sub-species’ up-listing to Threatened by COSEWIC in 2012. Subsequently, in 2013, BC’s Ministry of Environment published a recovery plan for coastal screech-owls outlining threats, recovery objectives, and knowledge gaps along with priority rankings and proposed timelines for each.
In 2016, wildlife biologist and screech-owl enthusiast Jared Hobbs asked if I’d be interested in a project to assess population trends of the kennicottii screech-owl. Despite warnings that coastal screech-owl numbers were already too low to inform on-going trends, I jumped at the opportunity to learn more about screech-owls and help bring together and review the information others had collected over the previous 20+ years. By 2016, a variety of monitoring efforts (Christmas Bird Counts, multi-year surveys, and monitoring of occupied territories) painted a consistent picture of widespread declines within BC’s south coast region (including areas from the lower mainland through Squamish Valley and Pemberton) and on southeast and eastern Vancouver Island (from the Victoria area up through Campbell River). In these areas, where most of the historic monitoring had been done, the remaining owls represented a small fraction of numbers observed in earlier decades.
Bigleaf Maple and Red Alder are dominant in the riparian systems that were historically thought of as good Western Screech-Owl habitat along the south coast of BC. (Photo: Toby St. Clair)
Re-assess what we know
The Report
170310_MFNRO WSOWke Report.pdf
Following submission of a winning bid to conduct the review and analysis of existing data to inform future coastal screech-owl monitoring, Jared and I quickly set to work validating and building on a database of historic detections that he and others had been maintaining and contributing to through a patchwork of funding and volunteer efforts in previous years. We enlisted the help of Jeannine Randall, working on her MSc at UNBC at the time, to solicit records and survey data from wildlife biologists who’d worked on coastal screech-owls and raided all the usual publicly available data sources (eBird, NatureCounts, etc.). I was impressed and encouraged to see so many positive responses to these requests. There was little hesitancy in sharing the information that so many had invested their time and resources to collect. These collaborative attitudes have been key in moving our state of knowledge on coastal screech-owl forward and I’m grateful to all those who contributed and to MFLNRORD for directing the effort. We completed a compilation and analysis of records and provided recommendations for future monitoring to MFLNRORD in 2017.
Our review of data compiled re-affirmed that dramatic declines in Western Screech-Owl populations in coastal BC had already occurred and future declines would be relatively subtle and challenging, if not impossible, to detect. In most of the areas from which we had recent survey data, few screech-owls were detected. Less than 20% of potentially suitable territories were occupied in most cases. In more recent years, these occupancy rates had dropped under 10% in many areas. That same year, in 2017, coastal screech-owls were up-listed to Threatened under SARA, thereby gaining legal protections and the promise of future critical habitat designations.
While the low occupancy rates we reported in 2017 limited our ability to assess trends in the areas where monitoring efforts had been focused previously, we assessed the compiled data to inform recommendations to maximize the value of future survey efforts. A key recommendation that PMRA is taking forward is to apply consistent survey methods that allow for determination of detection rates (e.g., repeated surveys during consistent times of year). Previous survey work was done to meet a variety of objectives and, consequently, has led to the collection of data under varying assumptions and constraints. Almost 20 different study programs conducted screech-owl surveys in BC between the late 90’s and 2016, and many of these yielded multiple years of survey data. However, few of these studies were conducted in a way that allowed determinations of the rate at which owls are detected during surveys when present which is required for “apples to apples” comparisons across studies.
By 2017, 933 coastal screech-owl records had been validated and were compiled into the provincial database. Since then, this number has grown to over 1,100. Across the province, almost all coastal screech-owl detections were reported at elevations under 500 meters (see figure below), which we expect are due to deeper snowpacks at higher elevations.
The lions share of records (68%) compiled in the 2017 database were reported from the lower mainland, valleys of the southern coastal range, and southeast Vancouver Island (see figure below). While these records and those collected since 2016 have helped identify many important habitat types and geographies for coastal screech-owl, the inference that can be drawn is limited without complimentary measures of survey effort. In other words, with occurrence records alone it’s difficult to know if a greater number of detections in any particular area or habitat type represents more owls or rather than greater survey effort.
Back to the bogs
The second report
200422_WESOke_2020_UpdatedOccupancyAnaly.
Given the low likelihood of detecting further change in areas surveyed the most in previous years, and following one of the recommendations from the 2017 report, MFLNRORD decided to focus subsequent monitoring efforts on habitat types with potential to support higher owl densities. Following up on the relatively high occupancy rates that Jeremiah had documented on the central coast, Jenna Cragg at MFLNRORD directed surveys on northern Vancouver Island within similar bog habitats and nearby low-productivity forests. These surveys, conducted by Bernard Schroeder and Guy Monty, yielded some of the highest densities of coastal Western Screech-Owl ever reported. MFLNRORD has since conducted 4 years of surveys in these and adjacent habitats using consistent methods that allow determination of detection rate and, thus, comparison with survey data from other parts of the province. Analyses of the first two years of the north island survey data found the presence of coastal screech-owl was negatively associated with barred owls, but the two species did co-occur in some areas. Additionally, these analyses found that detection rates of Western Screech-Owl increased in the absence of barred owls and in areas with higher densities of Western Screech-Owl.
While it’s encouraging to know that bog habitats may provide a refuge for screech-owls and some preliminary evidence suggests certain ancient forest types may similarly support these owls, it’s also important to keep our finger on the pulse of coastal screech-owl populations in habitats and geographies where declines may be continuing. There are some indications of continued declines in the Squamish and Pemberton Valleys where Greg Fergusson and others have collected valuable monitoring information in past years which could be compared to the results of future monitoring. Furthermore, some fundamental gaps remain in our understanding of coastal Western Screech-Owl beyond densities and trends in records. To determine the long-term viability of screech-owls in any area, high density or otherwise, we would need evidence that reproductive rates are sufficient to maintain those levels. Additionally, estimates of density and overall population size should also be informed by habitat specific home-range size which, in most cases, we don’t know. Current estimates of home range size are primarily based on the interior sub-species and sparse data from habitats that are distinct from both bog and ancient forest.
Our knowledge of coastal Western Screech-Owl suitable habitat and how to effectively survey and monitor them has progressed substantially since 2016. We have verified the importance of bog habitat following up on Jeremiah’s findings from the central coast, identified other potential refuge habitats in ancient cedar forest ecosystems and launched monitoring studies in those habitats as well.
The advancement of data collection and monitoring efforts in these and other habitats in future years will be essential to inform effective management of coastal Western Screech-Owl. Use of consistent survey methods, allowing determination of detection rates, should also allow us to explore other questions that have proven difficult to answer with the data available to date. For example, what is the relative importance of Barred Owl and habitat destruction, how do these factors interact, and under which circumstances (habitat types, forest management strategies) can the two owls co-exist? There are many more questions that we are anxious and excited to ask and with the coordinated efforts of PMRA volunteers and biologists across the province, we are hopeful that we will soon have the data we need to answer!
To wrap things up here, I’d like to again extend my thanks to everyone who has contributed their time and mental energy to our current state of knowledge on coastal Western Screech-Owl. Please engage with us if you have ideas that may not have been considered or misrepresented here or in the coastal screech-owl literature in general.
Document Library
I’ve referenced several status reports in this documents. Please see the “Resources” page on this site for access to these documents!
