There is strong evidence that climate change has caused many species to shift their geographical distributions as suitable habitat changes both spatially and temporally. Determining mechanisms behind distributional shifts and understanding why some species are shifting while others are not, may offer clues about adaptive capacity and species persistence into the future. The research presented here examines shifts and variability in species’ distributions in a community of birds in the Great Basin, USA using a long-term data set on avian abundance and novel occupancy and abundance modeling techniques.
Chapter one examined within breeding season elevational movement of 25 species of birds across two subregions of the Great Basin. This chapter examined the hypothesis that some species of birds disperse upslope as the breeding season progresses to track the distribution of higher-quality, late-season habitat as lower elevations become relatively hot and dry. Through the use of multinominal N-mixture models I examined 25 bird species over 7 years in two distinct regions of the Great Basin. I found evidence of upslope elevational dispersal in six species, and evidence of downslope elevational dispersal in one species, Green-tailed Towhee (Pipilo chlorurus). The results largely were consistent with the idea that environmental heterogeneity can drive dispersal. Changes in availability of six of the seven species (all except Broad-tailed Hummingbird [Selasphorus platycercus]) that dispersed within the season were associated significantly with shifts in primary productivity and food. The results of this chapter add to a growing body of research suggesting that within-breeding season dispersal is much more common than previously thought.
Chapter two assessed shifts in the elevational distribution of birds from 2001-2020. Although montane species are generally predicted to respond to climate change via upslope elevational movement, many populations and species are not moving in synchrony with increasing temperatures. This chapter examined 32 species of birds for evidence of elevational shifts at the upper and lower 25% of their elevational distribution as well as across the full elevational distribution. The elevational distributions of 19 species shifted, and the four shifts along the full elevational gradient were downslope. About half (46%) of the distributional shifts at the lower or upper elevational edges were upslope. Chapter two found evidence that elevational shifts in bird distributions may be a response to climate change, a signal detected over a relatively short time series (9 and 19 years).
Finally, chapter three examined associations of bird species with two different types of vegetational traits, plant physiognomy (characterized by functional groups) and plant floristics (characterized by plant species), in five biogeographically distinct subregions of the Great Basin. I hypothesized that plant physiognomy was significantly associated with bird occupancy across the Great Basin, while plant floristics were associated with occupancy within an individual subregion. This chapter found considerable variation among subregions with respect to which covariates were significantly associated with occupancy, and that the number and strength of bird-vegetation associations varied substantially between subregions. The results of this work suggest that for many bird species, vegetational associations are not transferrable across subregions, and that there is distinct geographical variation in vegetational preferences for some Great Basin bird populations.
