COOPER, S.J.*; BLEWETT, L.: Ventilatory Oxygen Extraction in Relation to Ambient Temperature in Black-capped Chickadees

We examined the effects of ambient temperature (T_a) on metabolism (VO₂) and ventilation in winter-acclimatized black-capped chickadees. We wanted to determine the relationships between VO₂ and respiratory frequency (f), tidal volume (V_T), and ventilatory oxygen extraction (EO₂). In addition, we tested the hypothesis that at low temperatures birds reduce ventilatory heat loss by increasing EO₂. Chickadees were captured in winter (Dec-Feb) and tested on the day of capture. We measured VO₂ and ventilation on chickadees exposed to ambient temperatures from 30^o to -15^o in air and at -9^oC in a 79% helium/21% oxygen gas mixture (helox). We analyzed data using linear regression and locally weighted scatterplot smoothing (LOWESS). The LOWESS technique makes no assumption about the form of the underlying distribution and is useful in looking for changes in the slope of a line. Increased VO₂ below thermoneutrality was supported by a linear increase in V_T, a linear decrease in f, and a linear increase in EO₂. Using LOWESS, the relationship between f and VO₂ and EO₂ and VO₂ showed abrupt changes in slopes at approximately 50% of VO_2sum. In other words, f and EO₂ are relatively steady (slope approximately zero) until a chickadee reaches 50% of summit metabolism (VO_2sum). At 50% of VO_2sum, f begins to decrease linearly and EO₂ increases linearly with increasing VO_2sum. Ventilatory heat loss from chickadees did not decrease substantially due to increased EO₂ until a bird was near VO_2sum.