NORTHERN SAW-WHET OWL MIGRATION MONITORING 2018 Annual Report

Transcription

1 NORTHERN SAW-WHET OWL MIGRATION MONITORING 2018 Annual Report Rocky Point Bird Observatory

2 TABLE OF CONTENTS Introduction... 2 Study Area and Methods... 2 Banding Protocol... 2 Rocky Point... 3 Pedder Bay... 3 Barred Owls... 4 Results... 4 Rocky Point Results... 5 NSWO RESULTS... 5 BDOW RESULTS... 5 Pedder Bay Results... 6 NSWO RESULTS... 6 BDOW RESULTS... 6 Discussion... 7 Conclusion and Recommendations... 8 Acknowledgements... 9 References

3 INTRODUCTION The Northern Saw-whet Owl (Aegolius acadicus) is a small, migratory raptor that is common and widespread. However, its migratory lifestyle makes population trends difficult to identify with standardized surveys. This owl has been monitored extensively in Eastern North America since the 1950 s but little is known about the western population and its migration patterns. The goal of this project is to collect data on the Northern Saw-whet Owls (NSWO) to contribute to a better understanding of the movement of the western population, and to increase public awareness of conservation issues regarding owls on Vancouver Island, British Columbia. In 2002, Rocky Point Bird Observatory, (RPBO) initiated the Nocturnal Owl Migration Monitoring Project with the aim of gathering more information on western populations of NSWO. Following protocols established by Project Owlnet ( RPBO began monitoring during Fall migration at Rocky Point Ammunition Depot. In 2014, a second constant-effort site was opened at the Pedder Bay Marina providing, in addition to monitoring, a training site for volunteers and an educational opportunity for the community. Having both banding locations allows for better monitoring of the species and the opportunity to note differences between the two sites. A total of Northern Saw-whet Owls have been banded between the Rocky Point and Pedder Bay stations since this project s inception. Owls banded at these two stations have been recaptured or bands recovered in British Columbia, Washington, Idaho, California, Alberta and Saskatchewan. Owls banded at other sites in British Columbia, Washington, California, and Alberta have been recaptured at our stations. A total of 710 Northern Saw-whet Owls were banded in 3856 net hours during the 2018 Nocturnal Owl Monitoring Project. STUDY AREA AND METHODS Banding Protocol The banding protocol and the standardized methodologies used at both stations were adopted from Project Owlnet. Banding commenced on 15 th September 2018 at both Rocky Point and Pedder Bay and ran until 31 st October Of the 47 nights of potential banding, five were lost to inclement weather between both Rocky Point and Pedder Bay, and an additional 19 nights had reduced hours due to inclement weather. Due to hours lost to weather, staffing, deer, and other factors, Rocky Point had net hours lost and Pedder Bay No nights were lost to Department of Defence closure or operation at Rocky Point in Owl monitoring began one-half hour after sunset, at which point the mist nets were opened and the audio lure was turned on. The nets remained open for six consecutive hours unless a closure was necessary due to poor weather or predator threats. Every 20 minutes, each net was checked for owls and if found they were carefully extracted, and each placed in a bird bag. The bag was securely closed, and a numbered clothes peg relating to the net was attached. 2

4 The bander then banded the owl with a numbered aluminum band on the right leg, and obtained the following morphometric data: body mass, wing chord, tail length, body condition, number of tail stripes, molt presence, eye colour, foot colour, and bill-tip colour. The net number was also recorded. The sex of NSWO was determined by a discriminant function analysis of mass and wing chord that was developed by Project Owlnet. Owls that were in the overlap range between the two sexes using this function were identified as unknown sex. The age of the owl was determined by the molt pattern on the primary and secondary flight feathers. Molt was recorded for each second-year (SY) or older owl. Molt patterns were pre-determined and outlined by Pyle (Pyle, P. 1997) and Project Owlnet ( To improve the accuracy of aging, a UV-A light was used to better observe the molt patterns. After the data was taken from the individual, is was released away from the banding station in a release box. If a NSWO was recaptured the same night it was originally banded, only the capture time, net number and weight were recorded. Rocky Point The Rocky Point banding location is situated near the southernmost tip of Vancouver Island, on the land owned by the Department of National Defense. Access to the site is tightly restricted due to the sensitive nature of military activities. The owl mist nets are located southeast of the banding station and consist of eight mist nets (12m x 2.6m), seven of which are owl nets (60mm square net pattern), and one of which is a passerine net (30mm square net pattern). The mist nets are arranged in a loose triangle with an audio lure in the center, surrounded by an array of peripheral nets. Pedder Bay Figure 1:Three generations of feathers on a NSWO (photo by: Rebecca Golat) Pedder Bay is located approximately four kilometres northwest of the Rocky Point station beside the road to the Pedder Bay Marina. The owl banding nets are located southeast of the banding station and 3

5 consist of seven mist nets (12m x 2.6m), five of which are owl nets (60mm square net pattern), and two of which are passerine nets (30mm square net pattern). In addition to gathering data on migration corridors and owl abundance, the goals of establishing the Pedder Bay site were to increase visitor access to banding operations and to allow more volunteers the opportunity to observe and gain owl banding and extraction experience. Barred Owls The Barred Owl (Strix varia) (BDOW) is a relatively recent inhabitant of Vancouver Island, having spread from the east coast over the last hundred years. The first BDOW recorded in the Victoria area was in November A generalist in terms of habitat preferences and prey, the Barred Owl is suspected to have impacted other owl populations and is a known predator of the Northern Saw-whet Owl. Although Barred Owls were not targeted, they Figure 2:Map of banding stations (google maps) were sometimes captured in the nets. When caught, the procedure followed was similar to the banding of the NSWO. A numbered aluminum band was placed on the right leg and the body mass, wing chord, tail length, age and sex were recorded. Age was determined using moult pattern an outlined in Aging Barred Owls in Rehabilitation Setting (Berry, 2013). Because of potential injury to NSWO, captured BDOW were held in portable kennels until the end of the banding session. Hatch-year BDOW, with prior permissions obtained from the Ministry of Environment and the Department of National Defense, were relocated in a suitable location well away from the banding sites. Adult (After Hatch-year) BDOW were released a short distance away from the banding operations. RESULTS In the 2018 season 710 Northern Saw-whet Owls were banded (444 at Rocky Point and 266 at Pedder Bay). An additional 66 Northern Saw-whet Owls previously banded were recaptured (40 at Rocky Point and 26 at Pedder Bay). The recapture figure does not include owls that were recaptured on the same night they were banded. Total net hours between both stations for the season was 3856, with 2077 at Rocky Point and 1779 at Pedder Bay. The capture rate for Rocky Point was.214 NSWO per net hour and.150 at Pedder Bay for a total of.184 NSWO per net hour between the two stations. A total of seven Barred Owls were banded and two recaptured. 4

6 Barred Owls were the only predator of NSWO in 2018 causing five fatalities, four at Rocky Point and one at Pedder Bay. Only one injury occurred for the season which was at Rocky Point. The owl was taken into veterinary care, treated and released NSWO were lost to predation for every 100 captured, which is higher than.59 in 2017 and very similar to.76 in 2016 (Rpbo.org, 2017). The incident rate (injuries and fatalities) was.85 per 100 captures was lower than 1.03 in 2017 (Rpbo.org, 2017). Rocky Point Results NSWO RESULTS A total of 444 new NSWO were banded at Rocky Point accounting for 62.5% of the season total across both stations. 97 (21.85%) owls were aged hatch-year, 276 (62.16%) second-year, 24 (5.41%) third-year, 6 (1.35%) after-hatch-year, 39 (8.78%) after-second-year and 2 (0.45%) after-third-year. Of the 444 Northern Saw-whet Owls banded at Rocky Point, 219 (49.32%) were identified as female, 46 (10.36%) were identified as male, and 179 (40.32%) were identified as unknown. Table 1. AGE BREAKDOWN OF NSWO AT ROCKY POINT STATION HY SY TY AHY ASY ATY TOTAL % 62.16% 5.41% 1.35% 8.78% 0.45% SEX BREAKDOWN OF NSWO AT ROCKY POINT STATION Male Female Unknown TOTAL % 49.32% 40.32% Table previously banded Northern Saw-whet Owls were recaptured At Rocky Point, not including 22 samenight recaptures. 20 NSWO were same-year recaptures, and seven were foreign recaptures. Two recaptured NSWO were originally banded in 2015, one as a hatch-year and one as a second-year at Rocky Point making them known fourth-year and fifth-year owls respectively. One was originally banded as a hatch-year in 2014 making it another fifth-year owl. Of the foreign recaptures, two were from Bainbridge Island, WA, two from near Lesser Slave Lake, AB, one from Tatlayoko Lake, B.C, and two owls whose original banding locations were undetermined at the time of submission of this report. BDOW RESULTS A total of six new BDOW were banded at Rocky Point. One bird captured as a HY in 2017 and released in Sooke was recaptured. Of the six BDOW banded, five were male and one was undetermined based on measurements. Two were aged as hatch-year birds and four were after-hatch-year. The recaptured bird was a second-year female which had been transported to a location 15 km away in Sooke for release. 5

7 Pedder Bay Results NSWO RESULTS A total of 266 new NSWO were banded at the Pedder Bay station, accounting for 37.5% of the season s total. There were 26 recaptures at this site. 43 (16.23%) owls were aged as hatch-year, 172 (64.91%) second-year, 10 (3.77%) third-year, 4 (1.51%) after-hatch-year, 35 (13.21%) after-second-year and 1 (0.38%) after-third-year. Of the 266 Northern Saw-whet Owls banded at Pedder Bay, 138 (51.88%) were identified as female, 23 (8.65%) were identified as male, and 105 (39.47%) were identified as unknown. AGE BREAKDOWN OF NSWO AT PEDDER BAY HY SY TY AHY ASY ATY TOTAL % 64.91% 3.77% 1.51% 13.21% 0.38% Table 3 SEX BREAKDOWN OF NSWO AT PEDDER BAY Male Female Unknown TOTAL % 51.88% 39.47% Table 4 26 previously banded Northern Saw-whet Owls were recaptured, not including four same-night recaptures. 19 NSWO were same-year recaptures, and one was a foreign recapture, banded in 2017 at Forest Ranch, CA. Two recaptured NSWO were originally banded as second and third-year owls in 2015 at Rocky Point, making them known fifth-year and sixth-years respectively. One was originally banded as a hatch-year in 2014, making it a fourth-year owl. BDOW RESULTS One new BDOW was banded and two were recaptured. The newly banded BDOW was a hatch-year male. The first recapture was a after-second-year female, and she was caught again later in the season. The second recapture was a second-year male from Rocky Point, which had been transported to a location 24 km away in Saanich for release in This is the greatest distance for a BDOW released after capture by RPBO and subsequently recaptured at either Rocky Point or Pedder Bay. 6

8 DISCUSSION Northern Saw-whet Owls populations are generally believed to follow in a four-year cycle (Swengel and Swengel 1995), however that cycle has broken down at our sites in recent years). Based on the results of 2016 and 2017, predicting the return for 2018 was difficult was expected to be a peak year and 2017 a crash year of a typical four-year-cycle. Instead, the opposite happened. Assuming 2017 was the peak of the cycle, a crash was expected for 2018, and to some extent that is what was seen. However, the total number of recaptures was similar to last year, despite the much lower numbers overall. In 2017, not including same-night recaptures, 67 NSWO were recaptured and in 2018, 66 NSWO were recaptured. The hatch-year owl numbers this year were very low. Out of all hatch year birds we've caught since 2012, this year was by far the lowest with 140 banded only accounting for 19.7% of all the owls banded between the two stations. The second lowest was 2013 having 37.3% of banded owls being HY. Crash years have a lower percentage of hatch year owls but this this year s was much lower then all previous years. Although the number of NSWO caught in 2018 was considerably lower than that in 2017, a large number of NSWO returned that had been banded last year. 21 individuals that were banded in 2017 returned in 2018, as did eight individuals banded in years Some notable returning individuals include two fifth-year and one sixth-year owls. There were also many owls banded and recaptured between stations. Pedder Bay caught 10 NSWO originally banded at Rocky Point in were recaptured at Rocky Point that were originally banded at Pedder Bay. Some of these recaptures happened in the same night as the owl was banded, demonstrating that the owls are not just moving in one direction, and also that there is considerable movement even for one night. The number of foreign recaptures (owls banded at another station, then recaptured by RPBO) was eight, which was remarkable considering that in all previous years, a total of 9 foreign birds had been recaptured. One was recaptured from the station in Tatlayoko Lake, B.C (the fourth we have recaptured from this site) and two were recaptured from Bainbridge Island, WA, (second and third recaptured) which suggests that at least some of the NSWO follow a migratory path. It also supports the need for more banding stations west of the Rocky Mountains to better determine migratory patterns. Three of the recaptures were from outside of B.C, two from Alberta and one banded in Forest Ranch in California. The NSWO recaptured from Forest Ranch is the first recaptured NSWO ever for that station. The 2018 season presented a different scenario than 2017, due the different volume and age of the owls. Although the number of NSWO was considerably lower than last year, more time was required on average with each bird to complete the aging process. Because of the large volume of older owls, volunteer banders and helpers were able to gain a lot of experience in ageing NSWO. Handling predation was a topic of extra concern after the 2017 season. In 2018 multiple tactics were employed, including more frequent net checks when BDOW were detected and using extra caution when releasing NSWO. Through these efforts there was a lower rate of incidents than 2017, 7

9 unfortunately the fatality rate was higher, the cause for the increased fatality rate is uncertain but the high percentage of AHY BDOW may have had an impact. For the first time since the project s inception, a passerine was caught and banded during the nocturnal owl banding. A Spotted towhee (Pipilo maculatus) was caught at midnight on October 3rd at Rocky Point. CONCLUSION AND RECOMMENDATIONS The 2018 banding year provided very different results to those of Where 2017 saw a very large number of newly banded owls (1848), 2018 saw less than half of that with only 710 new owls banded. However, despite the lower number of new bands, there were a similar number of recaptures in 2018 compared with 2017 (66 compared with 67). A suspected reason for the lower number of new bands is that the breeding grounds had unfavourable conditions for raising young. This further supports the claim that more banding stations are required on the west coast of North America to better understand the movement of NSWO. Recommended action for 2019 is to continue the Nocturnal Owl Migration Monitoring Project to further our understanding of the western population of Northern Saw-whet Owls. As one of the limited number of Northern Saw-whet Owl banding locations on the west coast, the Rocky Point Bird Observatory contributes valuable data towards understanding the fall migration movement of the western population of NSWO. Understanding the geographic origin of the owls captured at Rocky Point and Pedder Bay, via stable isotope analysing work or metals analysis could help us figure out the origins of the birds. Motus or satellite transmitters would better follow their routes and would provide valuable insight to the movement of NSWO. Funding and approval from the Science Committee at Rocky Point Bird Observatory will be key in gaining the technologies and staffing required to accomplish this. Reduce the risk of predation by BDOW includes adjusting the net monitoring frequency when BDOW have been detected and performing detailed scans on the area prior to NSWO release. While employing these tactics in 2018 the total number of BDOW and incident rates were lowered. In addition, further work to test the net sensor concept should be taken, subject to availability of technical support. Deer are another nuisance that affect banding efforts at both stations. Several nets were destroyed by deer foraging in and around the mist nets at the Pedder Bay station. No owls were harmed in the process, but valuable time and resources were lost by having to remove and replace the destroyed nets. A plan needs to be put into place to discourage deer from entering the banding area. Such plans could include stationing someone in the net lanes when deer are known to be in the area. 8

10 ACKNOWLEDGEMENTS The Northern Saw-whet Owl project was initiated by Paul Levesque in Katie McCreesh and Rebecca Golat served as the banders in charge for this project, and were assisted by banders Ann Nightingale, Wallace Reid, Collin Jennings, Donna Talluto, Kylli Morgan, Acacia Spencer-Hills and Jannaca Chick. A special thank you to Jo and Mike Motek, the project coordinators and Ann Scarfe, the volunteer coordinator. This project would not be possible without the dedicated group of volunteers that have put in 2420 hours of fieldwork to make the Nocturnal Owl Monitoring project what it is today. All data from this project have been submitted to the Canadian Wildlife Service for inclusion in their database and submission to the Bird Banding Laboratory of the U.S. Geological Survey. Figure 3: NSWO (Photo by: Rebecca Golat) 9

11 REFERENCES Projectowlnet.org. (n.d.). Age Determination. [online] Available at: [Accessed 4 Dec. 2018]. Pyle, P Identification Guide to North American Birds Part I. Slate Creek Press, Bolinas, CA. Rpbo.org. (2017). NORTHERN SAW-WHET OWL MIGRATION MONITORING. [online] Available at: [Accessed 4 Dec. 2018]. Berry, G. (2013). Aging Barred Owls in Rehabilitation Settings. [online] Available at: Settings-Autosaved.pdf [Accessed 4 Dec. 2018]. Swengel, A. B. & Swengel, S. R Possible four-year cycle in amount of calling by Northern Sawwhet Owls. The Passenger Pigeon, 57: