COS 215-3 - Nesting sites and declining summer snowpack co-limit habitat for one of North America’s highest-elevation breeding birds, the Sierra Nevada Gray-crowned Rosy-Finch (Leucosticte tephrocotis dawsoni)

Abstract: Diminishing mountain snowpack worldwide is anticipated to affect many alpine organisms. Western North America’s snowpack is expected to decline by up to 80% by 2100. In alpine ecosystems with brief breeding seasons, species must effectively track spatiotemporal resource variations to align their reproductive efforts with periods of peak resource availability. Here, we examine potential effects of snowpack decline by investigating how hypothesized key habitat features, including snow, influence breeding site occupancy by an alpine specialist songbird. The Sierra Nevada Gray-crowned Rosy-Finch (Leucosticte tephrocotis dawsoni, “Rosy-Finch”) breeds at the highest elevations within its narrow geographic range of the Sierra Nevada and White Mountains, California, USA. During the summer breeding period, Rosy-Finch diets consist of snow-trapped insects, emerging aquatic invertebrates and plant foods in the alpine ecosystem. We hypothesized that breeding habitat availability for Rosy-Finches is influenced by a combination of climate-influenced features, including late-lying snowfields, absence of woody vegetation and available cliff-nesting sites. We selected 27 sites in a stratified random design across the Rosy-Finch's estimated range based on elevation, topographic roughness, and alpine vegetation in California’s Sierra Nevada and White Mountain ranges. In 2018-2022 we applied point count distance sampling to measure occupancy relative to environmental variables, including proximity to snowpack and alpine lakes (foraging resource), cliffs (nesting habitat), and woody vegetation. We constructed a binomial N-mixture model to estimate the detection and occupancy probabilities of Rosy-Finches and relate them to environmental variables across years.



Across five breeding seasons, probability of Rosy-Finch site occupancy remained stable throughout their distribution (0.65-0.68). Rosy-Finch presence was positively associated with proximity to cliffs, snow patches, and alpine lakes and negatively associated with the presence of woody vegetation. Notably, species occurrence increased steadily with elevation. We observed no significant impact of interannual snowpack or climate variability on occupancy, although both may have affected breeding success. The strong interconnection between high-elevation cliffs and snowpack in mountainous regions highlights the critical role of these alpine features in determining Rosy-Finch breeding habitat. Our findings underscore the need for ongoing monitoring and research to better understand interactions between environmental factors and breeding success in Rosy-Finches, and to inform targeted conservation actions to ensure long-term persistence of this species in the face of ongoing environmental change.