The American Kestrel & Cavity-Nesting Owls
American kestrels (Falco sparverius) are considered one of the most numerous falconiform species in North America, but some research demonstrates that many populations are declining. One of the limiting factors for these birds is the availability of nesting sites - kestrels do not build their own nests, but rely on existing cavities, usually excavated by larger woodpeckers. Similarly, Boreal owls (Aegolius funereus), Northern Saw-whet owls (Aegolius acadicus), and the Northern pygmy owl (Gaucidium gnoma) all nest in cavities too, and they are not well studied in the Western Boreal region of Alberta. Forested habitats are undergoing rapid change due to the cumulative effects of natural disturbances, resource extraction and global change.
Objectives
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Assess presence and habitat preference using nest boxes and ground-based surveys
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Document food composition, breeding success and productivity
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Investigate nest site fidelity, recruitment, post-natal dispersion, and migration
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Compare breeding performance of raptors nesting in predominantly forested areas to those nesting in the agricultural/developed landscapes
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Build and maintain a long-term database on habitat preference and nesting success.
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Collaborate with organizations and institutions to improve understanding of these species’ biology and ecology.
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Inform land managers, other stakeholders, and the public about the project
Nest Box Experiments
Fortunately, all of the species mentioned above readily accept man-made wooden nest boxes, and if placed in suitable habitat types, they can be an effective conservation tool and increase the breeding population. Nest boxes also allow us to learn about the breeding biology of these birds: we can monitor trends in laying dates, clutch size, and nesting success. Additionally, by marking adults and nestlings with bands issued by the Banding Office, we can track longevity, nest site fidelity, recruitment, migration routes, wintering grounds, and many more aspects of the biology and ecology of these species. We build nest boxes of rough-cut spruce boards (or other untreated wood product), following a box design that offers adequate insulation and space for a brooding female and cached prey items. Boxes are estimated to last at least five years, with only minor repair or cleaning needed between the breeding seasons.
For kestrels, we place the boxes on single trees or tree clumps in pastures, free standing utility poles, or lumber secured to existing fence posts. For owls, we place the boxes in forests that are large enough to allow breeding of at least one pair of owls, and they are placed at least 200 meters from each other on live or dead trees between 3 - 6 meters high. From mid-March, we check every owl box at least twice to confirm occupancy and to cover the timing of both early breeders and possible replacement clutches. Kestrels arrive in April, so we start nest box checks in mid-April or when the first migrating kestrels are detected at the site.
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Initial visits consist of a quick glance into the box using a wireless observation device. Occupant species, eggs, cached prey, or any other signs of occupancy (e.g., feathers, additional nest material) are noted and future check dates are scheduled based on the initial findings. Occupied boxes are re-visited to assess fecundity and hatchling survival until the clutch fails or nestlings are at fledgling age (28 - 30 days old). We try to mark all nestlings 22 - 28 days after hatching to track dispersal, migration routes and recruitment into populations. Whenever possible, we try to capture adults as well, to find answers about dispersal, nest-site fidelity, survival, and migration.
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After the breeding season ends, all boxes are cleaned, repaired, or replaced as needed and made available for the next breeding period. Occasionally, nest predation could become a problem, and in such cases, boxes are relocated 50 -100 meters from the original location.
Ground-based Surveys
To detect the presence of owls, we organize one to two nocturnal owl surveys through March and late April. Initially, we conducted a desktop analysis of the study area using Google Earth to identify potential areas for owl surveys. We selected 12 -15 points that are at least 800 - 1000 meters from each other. At each location, starting half-an hour after sunset, we conduct a passive listening period of 10 minutes, divided in 10 one-minute intervals. All owls heard or observed are registered in the appropriate one-minute interval at the time of first detection. We also record any incidental species identified during the owl survey.
We obtain additional information about the movement and distribution of kestrels during the roadside raptor surveys used to assess the presence of diurnal birds of prey.