PHYSIOLOGICAL MODELING OF OXYGEN-CONSUMPTION IN BIRDS DURING FLIGHT

This study combines data on changes in cardiovascular variables with b ody mass (M(b)) and with exercise intensity to model the oxygen supply available to birds during flight. Its main purpose is to provide a fr amework for identifying the factors involved in limiting aerobic power input to birds during flight and to suggest which cardiovascular vari ables are the most likely to have been influenced by natural selection when considering both allometric and adaptive variation. It is argued that natural selection has acted on heart rate (fH) and cardiac strok e volume (VS), so that the difference in the arteriovenous oxygen cont ent (CaO2-CVO2) in birds, both at rest and during flight, is independe nt of M(b). Therefore, the M(b) exponent for oxygen consumption (VO2) during flight can be estimated from measurements of heart rate and str oke volume. Stroke volume is likely to be directly proportional to hea rt mass (M(b)) and, using empirical data, values for the M(b) coeffici ents and exponents of various cardiovascular variables are estimated. It is concluded that, as found for mammals, fH is the main adaptive va riable when considering allometric variation, although Mb also shows a slight scaling effect. Relative Mb is likely to be the most important when considering adaptive specialisations. The Fick equation may be r epresented as: (VO2)M(b)(z) = (fH)M(b)(w) X (VS)M(b)(x) X (CaO2 - CVO2 )M(b)(y), where w, x, y, z are the body mass exponents for each variab le and the terms in parentheses represent the M(b) coefficients. Utili sing this formula and data from the literature, the scaling of minimum V-O2 during flight for bird species with a 'high aerobic capacity' (e xcluding hummingbirds) is calculated to be: 166M(b)(0.77+/-0.09) = 574 M(b)(-0.19+/-0.02) X 3.48M(b)(0.96+/-0.02) X 0.083M(b)(0.00+/-0.05), a nd for hummingbirds (considered separately owing to their unique wing kinematics) it is: 314M(b)(0.90+/-0.22) = 617M(b)(-0.10+/-0.06) X 6.13 M(b)(1.00+/-0.11) X 0.083M(b)(0.00+/-0.05). These results are largely dependent on the cardiovascular values obtained from pigeons flying ne ar to the minimum power speed of 10 ms(-1), but would appear to provid e realistic values. Both the measured and the estimated VO2 for hummin gbirds appear to scale with a larger M(b) exponent than that for all o ther birds, and it is suggested that this is as a result of the larger M(b) exponent for flight muscle mass as the larger species of humming birds try to maintain hovering performance. It is proposed that estima ted VO2 for birds during flight, which is based on Mb in combination w ith estimates of fH and CaO2-CVO2, gives an indirect measure of relati ve aerobic power input and, when corrected for the estimated scaling i nfluences of the mechano-chemical conversion efficiency and lift gener ation with respect to Mb, may be a useful indicator of the relative ca pacity of the muscle to sustain power output and lift production durin g flight.