COS 205-5 - Understanding how spatial environmental heterogeneity influences population density and constrains habitat for a range-restricted alpine bird

Abstract: Accurate characterization of restricted animals’ distributions and abundances are crucial for both current conservation and anticipating future changes. Individual habitat use and fine-scale environmental determinants of varying animal density are two areas that can augment occupancy and coarse density approaches. The Brown-capped Rosy-Finch (BCRF, Leucosticte australis) has a limited breeding range in the Southern Rocky Mountains, USA, and may be disproportionately affected by climate changes due to its specialized alpine cliff-nesting breeding habitat, paired with disproportionate warming projections for high altitudes. Spatial heterogeneity within alpine areas means not all potential breeding habitat is likely suitable for BCRF. Our study goals for this understudied bird were to (1) measure population density and use it to estimate the range-wide population size and (2) investigate factors that influence varying density within breeding basins and throughout their range. We conducted breeding habitat surveys in the alpine in 2018-2020 using modified line transect distance sampling to produce a breeding range density estimate. We analyzed BCRF resource selection within breeding basins with three different analyses: (1) relating individual bird locations to habitat features hypothesized as important; (2) asking whether behaviors differed between alpine cover types; (3) comparing ground cover in 25m plots where BCRF were or were not observed. Our density estimate ranged from 16.1-20.9 BCRF/km^2 ( 95% confidence intervals overlapped between years). Probability of BCRF presence was positively associated with distance to cliffs and to snow. As elevation increased, probability of detecting a BCRF increased, within the range of 3422-4106m. Unlike other Rosy-Finch species, BCRF presence was unrelated to water resources such as alpine lakes. BCRF foraged predominantly on snow and vegetated areas and exhibited non-foraging behaviors in cliff and rock-dominated areas. BCRF were most likely to occur in talus, then scree, followed by snow; and less likely to occur in areas with dense vegetation. We found that BCRF occur at higher densities near potential cliff nesting sites, close to snow and rocky substrates, and are not uniformly distributed across their predicted breeding range. Potential effects of climate changes, including decreased winter snowpack, earlier snowmelt, and increased vegetation density and cover, may negatively affect BCRF in the future. Incorporating relationships of birds to specific habitat features improves understanding of both current and future BCRF breeding habitat use, distribution and abundance.