COOPER, S.J.*; SAME, D.R.: Ventilatory Accomodation Under Cold Stress in Seasonally Acclimatized Black-capped Chickadees

Black-capped chickadees undergo seasonal acclimatization that is characterized by an increase in both basal and summit metabolism in winter birds compared to summer birds. Little is known about how the avian ventilatory system accommodates the large changes in rates of oxgyen consumption (VO₂) between basal (BMR) and summit metabolism (VO_2sum). We measured ventilation and VO₂ under thermoneutral conditions and cold stress in seasonally acclimatized black-capped chickadees. Cold stress tests were conducted using a gas mixture of 79% helium and 21% oxygen (helox). Metabolic expansibility (VO_2sum/BMR) was 5.5 in summer and 6.7 in winter. For summer birds, the increase in VO₂ under cold stress was accommodated mainly by an increase in tidal volume (V_T). For winter birds, the increase in VO₂ under cold stress was accommodated by decreased respiratory frequency, an increase in V_T and a significant increase in ventilatory oxygen extraction (EO₂). The winter EO₂ of 68.3% was significantly higher than summer birds (42.5%) and is the highest value so far documented in birds. There were no other significant differences in ventilation between summer and winter birds under thermoneutral or cold stress conditions. Our data suggests that changes in EO₂ are involved with seasonal acclimatization to cold in black-capped chickadees.