3109Flying vertebrates are of interest to cardiovascular physiologists because their aerobic metabolic capability is about twice that of running mammals of equal size (Thomas et al., 1987). Because homing pigeons Columba livia combine a relatively small body size (allometric variation) with superior athletic ability (adaptational variation) and tractable behavior, they are an excellent animal model for investigating the upper limits of vertebrate cardiorespiratory adaptation and performance during intense metabolic stress.Over the years, much has been learned about the many aspects of the cardiorespiratory responses of birds to treadmill exercise (Bevan et al., 1994(Bevan et al., , 1995 and resting hypoxic stress (Maginniss et al., 1997;Novoa et al., 1991). Other studies have investigated various aspects of avian flight physiology that include metabolism and biochemistry (Christensen et al., 1994;George and John, 1993;Schwilch et al., 1996), thermoregulation and water balance (Adams et al., 1997;Carmi et al., 1993Carmi et al., , 1994Hissa et al., 1995;Giladi et al., 1997), and wing cycle and ventilation (Boggs et al., 1997a,b). However, only one in-depth cardiorespiratory study on an avian species during steady flight has been reported (Butler et al., 1977). Consequently, this central aspect of avian physiology remains poorly understood.Two reasons for the scarcity of detailed avian flight cardiorespiratory data are the need for specialized equipment (e.g. a wind tunnel) and chronic vascular cannulation techniques, which provide access to the bird's circulatory system while allowing normal behavior of the bird. Accordingly, the two primary objectives of this study were: (1) to develop chronic cannulation techniques and blood sampling procedures for the pigeon in order to routinely obtain arterial and mixed venous blood samples from both resting and flying birds, and (2) to comprehensively characterize the basic cardiorespiratory parameters of our population of homing pigeons at calm rest and during steady wind tunnel flight under defined conditions. Materials and methods Experimental animals and housingHoming pigeons Columba livia L. to be investigated were of unknown sex and obtained from Dr Melvin Kreithen's outdoor loft at the University of Pittsburgh; mean body mass (± S.D.) was 340±45·g. The birds were housed in individual indoor cages at 21°C under a 12·h:12·h (light:dark) photoperiod with ample commercial pigeon food and water. All data reported in this study were collected between 09:00·h and 01:00·h. Screening and familiarization Resting studiesThe birds were first screened to determine whether they were We made detailed cardiorespiratory measurements from homing pigeons during quiet rest and steady wind tunnel flight. Our pigeons satisfied their 17.4-fold increase in oxygen consumption during flight with a 7.4-fold increase in cardiac output (Q) and a 2.4-fold increase in blood oxygen extraction. Q was increased primarily by increasing heart rate sixfold. Comparisons between our study and those from th...