Testing the myth of humid versus dry cold: birds do not care

At high air temperatures (Ta) atmospheric water vapor can markedly influence an animals heat dissipation by reducing evaporative cooling capacities. At low Ta however, the impact of increased water vapor on heat exchange has sparsely been investigated, despite human populations at northern latitudes insisting on feeling colder on days with higher humidity (i.e., humid-cold). Here, we aimed to investigate the humid-cold perception by determining whether Black-capped Chickadees (Poecile atricapillus) exhibited greater energy expenditure through increased heat loss under humid-cold conditions relative to dry-cold. We measured resting metabolic rates (RMR) of captive chickadees (n=10) during exposure to two Ta treatments, below freezing (Ta {approx} -7{degrees}C) and above freezing (Ta {approx} 10{degrees}C), with either dry or saturated air. We found that Ta substantially impacted RMR, with RMR {approx}1.5-fold greater at -7{degrees}C compared with 10{degrees}C. Conversely, humidity did not have a statistically significant impact on RMR at either Ta. Our data thus suggest that humidity does not significantly influence an individuals heat exchange with the environment at cold Ta. This presumably reflects the fact that cold air can hold minimal amounts of water vapor and any possible influence on heat loss is negligible. Instead, we suggest that a greater contributor to the humid-cold myth is the frequency of overcast days during which direct solar radiation is blocked. Summary statementBlack-capped chickadees significantly increased their resting metabolic rates at air temperatures below freezing relative to air temperatures above freezing, whereas humidity level did not significantly affect resting metabolic rates.