Olivia Ann Steinmetz, Timothy H Parker 

Tropical montane bird species may be vulnerable to climate change as they often have narrow elevational ranges that are expected to shift upslope in response to increasing temperatures. If highland endemics near mountaintops are forced upslope, these species may be especially at risk of local extinction. We investigated the elevational ranges of two species of montane cloud forest warblers, the Slate-throated Redstart (Myioborus miniatus) and the Collared Redstart (M. torquatus), in Costa Rica between 1980 and 2019. The Collared Redstart is a highland endemic that has been projected to go locally extinct from climate change. We examined 11,856 and 7,366 checklists from sites where the Slate-throated Redstart and the Collared Redstart, respectively, have been reported to eBird, a community science project managed by the Cornell Lab of Ornithology. As expected, we found that the two species’ elevational ranges were different from each other but overlapped, with the Collared Redstart showing a peak rate of detection at an elevation about 750 m to 1,050 m higher than the Slate-throated Redstart. More important, only the Slate-throated Redstart showed a shift upward in its elevation of peak detection over time. This inconsistent response to climate change reinforces the growing evidence that the impacts of climate change on tropical montane bird species are complex and difficult to forecast. Continuing to study the elevational ranges of tropical montane bird species is vital for our understanding of these birds’ responses to climate change.