Unstructured community science data reveal constriction of breeding distribution for a common montane bird across the Fennoscandian peninsula

The threat of climate change is particularly acute for species in arctic and montane habitats, where changes are happening the most rapidly. Species are generally expected to shift their ranges northward and upslope in response to changing climates, but actual measured shifts in species distributions have been nuanced and large quantities of data are needed to accurately assess shifts. The growing availability of unstructured community science data is an appealing solution to the problem of characterizing changes in species distributions, but these data contain known biases that must be overcome to draw strong inference. Here, we leveraged opportunistic, unstructured community science data from across the Fennoscandian peninsula over the last 45 years to evaluate the hypothesis that the breeding distribution of a common and iconic montane bird, the bluethroat (\textit{Luscinia svecica}), has shifted towards higher latitudes and elevations. We constructed non-detections of bluethroats using detections of 12 less notable 'background' avian species, which allowed us to analyze over 500,000 observations within a robust spatio-temporal occupancy modeling framework. We found that bluethroat occupancy has declined substantially over the past four decades across Fennoscandia. The largest absolute declines in occupancy probability took place in areas with higher occupancy in the early years of the study, but the largest relative changes in occupancy took place at low latitudes and low elevations at high latitudes. Our work demonstrates that even common and globally stable species are at risk of breeding habitat loss under the looming threat of climate change and that unstructured community science data, when used thoughtfully, can fill important knowledge gaps about species responses to global change.