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Abstract 

Black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees are common backyard birds native to the ŷ���ڱ���Ƶ Front-Range with broadly overlapping distributions in western North America. Black-capped chickadees are more common at lower elevations, and mountain chickadees are more common at higher elevations . The nests of these two species, which they construct in cavities, can be distinguished by either having more moss and plant material (black-capped chickadees) or more animal hair and feathers (i.e., “fluff;” mountain chickadees). I examined nests that I collected from field sites spanning an elevation gradient from 1600 to 3000 meters above sea level. I then investigated their thermoregulatory capacity and composition to understand if species or elevation had an effect on how well nests maintain heat. I found that there is a significant difference in thermoregulatory capacity between species, but that thermoregulatory capacity does not increase with increasing elevation. I also found that mountain chickadee nests, whose nests plateau at higher temperatures, have, on average, higher proportions of fluff than plant material compared to black-capped chickadee nests. As climate changes, particularly in high elevation mountain systems, understanding whether species have the capacity to rapidly respond is important. My results indicate that nest thermoregulatory capacity varies by species but not with elevation, and provides a baseline for understanding whether chickadees will alter their nests in response to warming temperatures. Adaptive responses may be unlikely given that chickadees do not build warmer nests at higher elevations and additional work should examine correlations between nest thermoregulation, climate, and the success rate of clutches.