Climate change drives range contraction and shapes species distribution in an alpine passerine: Caution required when comparing atlas data

The White-winged Snowfinch ( Montifringilla nivalis ), a specialist in alpine habitats, is highly vulnerable to climate change due to its reliance on high-elevation environments, which are experiencing accelerated warming, alterations in snow dynamics, and significant shifts in phenology, among other factors. These habitats, crucial for breeding and feeding, are often under-sampled due to their remoteness and difficult accessibility. Furthermore, they are shrinking, particularly at the species' range edges. Comparing data from the II and III Spanish bird atlases (1998–2002 and 2014–2018), we observed a significant decline in its breeding range, with the species disappearing from 17 grid cells (11 in the Cantabrian Mountains and 6 in the Pyrenees). However, direct comparison between the atlases is problematic because 30 grid cells surveyed in the II atlas were not sampled in the III, potentially leading to an overestimated decline. While eBird data from 2014–2018 partially mitigated the perceived decline, the same 30 grid cells have remained unsampled, further complicating a definitive assessment of the species’ status. Cool summer temperatures and low humidity, as indicated by the Topographic Wetness Index (TWI), were identified as the most influential environmental factors affecting the species' breeding distribution, based on distribution data from both the III atlas and eBird, analyzed through a presence-absence distribution model. Interpreting atlas data requires caution, particularly for species inhabiting remote or inaccessible areas. Alternative data sources, such as eBird, can support ongoing monitoring programs by providing additional insights into species distribution. However, even with eBird, certain hard-to-reach areas remain under-sampled, causing them to still be presumed unoccupied. Increasing monitoring efforts in these highly favourable but under-sampled regions is crucial to enhance conservation strategies for this declining alpine species.