Gyrfalcon prey abundance and their habitat associations in a changing Arctic

Arctic habitats are changing rapidly and altering trophic webs and ecosystem functioning. Understanding how species’ abundances and distributions differ among Arctic habitats is important in predicting future species shifts and trophic-web consequences. We aimed to determine the habitat-abundance relationships for three small herbivores on the Seward Peninsula of Alaska, USA by fitting data from 983 point counts (collected during 2019, 2021, 2022) with N-mixture models that account for imperfect detection. These herbivore species, Willow Ptarmigan [Lagopus lagopus], Rock Ptarmigan [L. muta], and Arctic ground squirrels [Urocitellus parryii]), are fundamental to tundra food webs, and primary prey for Arctic raptors including Gyrfalcons (Falco rusticolus). Second, we aimed to map herbivore densities within Gyrfalcon breeding territories. Third, we aimed to evaluate whether Gyrfalcons were more likely to occupy territories with higher prey densities using a multi-season occupancy model coupled with occupancy observations from helicopter surveys conducted during 2016-2022 at 97 Gyrfalcon territories. We found that male Willow Ptarmigan were more abundant in areas with greater cover of tundra, tall shrubs, and tussock tundra. Conversely, male Rock Ptarmigan were more abundant in areas with greater cover of sparse vegetation and tundra. Arctic ground squirrels were more abundant at higher elevations with greater cover of sparse vegetation and low shrubs. Willow Ptarmigan were widespread within Gyrfalcon breeding territories, whereas Rock Ptarmigan and Arctic ground squirrels had patchier distributions with few areas of high abundance. Lastly, Gyrfalcons were more likely to occupy territories with higher densities of Willow Ptarmigan and Arctic ground squirrels. As the Artic continues to warm, Rock Ptarmigan and Arctic ground squirrels may be vulnerable to ongoing shrub encroachment, whereas Willow Ptarmigan may benefit. By tying abundances of three prey to Gyrfalcon occupancy, our results contribute to understanding potential impacts on higher levels of this Arctic trophic web.

Methods
The study conducted multi-species point count surveys to detect bird species and Arctic ground squirrels over three years (2019-2022). Surveys began 30 minutes before sunrise and ended when at least ten points were surveyed or by noon. Each point was surveyed once. The surveys followed a time-removal approach, with specific time intervals acting as replicate survey periods. Observations were made for ten minutes, with a two-minute quiet period beforehand. The first detection of each individual within 400 meters was recorded, excluding subsequent detections of the same individual. Data collected included species, time, distance, bearing, group size, and sex. Individuals not actively using the area (e.g., flyovers) were excluded. Observers also measured temperature, wind speed, and direction at each point. Surveys were not conducted in winds over 24 km/h or in heavy rain or fog.

From 2016 to 2022 (excluding 2020 due to Covid-19), the study monitored 97 Gyrfalcon territories on nesting cliffs, conducting two surveys per territory each year between 1 May and 2 July. Surveys were performed using an R44 helicopter, following established protocols. A territory was considered potentially occupied if a Gyrfalcon adult, egg, or nestling was observed during the survey. Sixty surveys were not completed due to weather, and those were treated as missing data. The study also used prior surveys (2011-2015) to confirm whether territories were historically Gyrfalcon territories, as some were unoccupied or occupied by other raptors during the 2016-2022 period.

The study used vegetation data from the Arctic Boreal Vulnerability Experiment (ABoVE) to describe land cover on the Seward Peninsula. The ABoVE project classified vegetation into 15 types based on dominant vegetation derived from satellite imagery at a 30-meter grid resolution. The dataset from 2014 was used, and relevant habitat types for each species (Willow Ptarmigan, Rock Ptarmigan, and Arctic ground squirrels) were selected based on their preferred habitats. Vegetation types such as tundra, tussock, low shrub, and tall shrub, along with elevation, were chosen for the models. Data for each survey point included vegetation cover within a 0.64-km² area and mean elevation using a digital elevation model (DEM). Areas above 500 m elevation were excluded.

To estimate prey density inside each Gyrfalcon territory, we excluded cells within each territory with vegetation percentage values that fell outside the ranges used in our prey abundance models, and with elevations exceeding 500 m. We calculated the area size of each territory correcting for these removed cells. We summed expected density of each prey from the grid cells inside each Gyrfalcon territory and divided by the corrected territory area. For Willow Ptarmigan and Rock Ptarmigan, densities were expressed as males per square kilometer, whereas for Arctic Ground Squirrels, densities were reported as squirrels per square kilometer.