Nestling red-winged blackbirds exhibited two distinct metabolic reactions to lowering temperatures. One group, which maintained homeothermic conditions, elevated its metabolism linearly according to a drop in ambient temperature and the second decreased its metabolic response in a curvilinear fashion. Core temperatures showed that although there was a progression toward the establishment of homeothermy throughout the nestling period, the birds could and did drop their core temperatures when the temperature gradient was too great to maintain constant body temperature. In some instances it is felt that oxygen consumption, and thus metabolism, was lowered before body temperature was suddenly dropped. No differences in the rates of metabolism could be found between upland and marshland populations; however, differences between values of the respiratory quotient for the two populations were found (p < 0.001). In addition differences in R.Q. values were found between well-developed nestlings practicing homeothermy and those that became hypothermic in response to lowering environmental temperature.

The physical properties, including erythrocyte numbers, leucocyte numbers, and differential ratio, pH, pCO2, pO2, O2 saturation, haemoglobin levels, haematocrits, and mean cell volumes are recorded for the red-winged blackbird (Agelaius phoeniceus).

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