The proton conductance of isolated liver mitochondria correlates significantly with body mass in mammals, but not in ectotherms. To establish whether the correlation in mammals is general for endotherms or mammal-specific, we measured proton conductance in mitochondria from birds, the other main group of endotherms, using birds varying in mass over a wide range (nearly 3000-fold), from 13 g zebra finches to 35 kg emus. Respiratory control ratios were higher in mitochondria from larger birds. Mitochondrial proton conductance in liver mitochondria from birds correlated strongly with body mass [respiration rate per mg of protein driving proton leak at 170 mV being 44.7 times (body mass in g)(-0.19)], thus suggesting a general relationship between body mass and proton conductance in endotherms. Mitochondria from larger birds had the same or perhaps greater surface area per mg of protein than mitochondria from smaller birds. Hence, the lower proton conductance was caused not by surface area changes but by some change in the properties of the inner membrane. Liver mitochondria from larger birds had phospholipid fatty acyl chains that were less polyunsaturated and more monounsaturated when compared with those from smaller birds. Phospholipid fatty acyl polyunsaturation correlated positively and monounsaturation correlated negatively with proton conductance. These correlations echo those seen in mammalian liver mitochondria, suggesting that they too are general for endotherms.