Hypothermia during general anesthesia of avian species can be life threatening and can develop even when supplemental heat is provided. To compare the change in core body temperature in avian patients anesthetized with different anesthetic delivery systems, we monitored the core body temperature of 12 pigeons (Columba livia) during isoflurane anesthesia delivered by 3 commercially available anesthetic systems: a circle system attached to a mechanical ventilator producing heated air, a nonrebreathing circuit with a mechanical ventilator delivering nonheated air, and a Bain nonrebreathing circuit. Each pigeon was anesthetized with each of the 3 anesthetic delivery systems in a randomized complete crossover trial. Mean core body temperature decreased significantly with all anesthetic delivery systems, with no significant difference between the change in core body temperature at 45 and 90 minutes among the 3 systems. With all 3 systems, the core body temperature of all pigeons remained within the range of 36.2°C–42.0°C (97.2°F–107.6°F). Although the mean inspired gas temperature was significantly higher with the heated-air ventilator system than with the nonheated-air ventilator and Bain nonrebreathing circuit systems, this difference did not contribute to the maintenance of the core body temperature. No one system was superior in preventing hypothermia during isoflurane anesthesia of pigeons.