Aim: The duration of the electrocardiographic QRS complex is the time required for depolarization to spread throughout the ventricular myocardium. In the rank of birds, the duration of the QRS complex increases <10- fold, whereas body mass (BM) augments [asymp]100000-fold and heart mass (HM) increases >10000-fold. This study defines how the duration of the QRS complex is related to BM, heart rate (HR), and heart index (HI, the HM- to-BM ratio) in nonpasserine species of birds encompassing a 1000-fold BM range. Methods: The literature was searched for original data on QRS duration (ms), HR (bpm), BM (kg), and HI (%) in different avian species of 13 families and 10 orders. Values from several studies on the same species were averaged. BM of the birds (n=28) ranged from 0.106 to 90 kg. HR (n=32) ranged from 50 to 480 bpm. HI (n=16) ranged from 0.22 to 1.68%. The QRS duration (n=32) ranged from 13 to 60 ms. Results: The QRS duration scales positively with BM as QRS=24×BM^{0.18±0.03 (±SE, p<0.001)} (p<0.001). The dependence of the QRS duration on HR is described by the equation QRS=292×HR^{-0.45±0.12 (p<0.001)} (p<0.001). The QRS duration is independent of HI (QRS=24×HI^{-0.07±0.21 (p>0.05)}; p>0.05). On the basis of allometric equations that relate HM to BM in birds (Grubb 1983, Bishop 1997, Seymour & Blaylock 2000), we determined that the dependence of the QRS duration on HM is described by the equation QRS[asymp]15.7×HM^0.195. Conclusion: In nonpasserine birds, the duration of the QRS complex augments as HR decreases, and that more rapidly than as BW increases. The findings suggest that from small to large nonpasserine birds, a density of the Purkinje fibre system and/or Purkinje fiber–myocardial cell regions in the heart ventricles increases. Bishop, C.M. 1997. Philos Trans R Soc Lond B Biol Sci 352, 447-456. Grubb, B.R. 1983. Am J Physiol 245, H567-H572. Seymour, R.S. & Blaylock, A.J. 2000. Physiol Biochem Zool 73, 389-405.