Blood oxygen-carrying capacity is one of important determinants of oxygen amounts supplied to the tissues per unit time and plays a key role in oxidative metabolism. In wild vertebrates, blood oxygen-carrying capacity is most commonly measured with the total blood haemoglobin concentration (Hb) and haematocrit (Hct), which is the volume percentage of red blood cells in blood. Here, I used published estimates of avian Hb and Hct (nearly one thousand estimates from 300 species) to examine macroevolutionary patterns in blood oxygen-carrying capacity of blood in birds. Phylogenetically-informed comparative analysis indicated that blood oxygen-carrying capacity was primarily determined by species distribution (latitude and elevation) and morphological constraints (body mass). I found little support for the effect of life history components on blood oxygen-carrying capacity, except for a positive association of Hct with clutch size. Hb was also positively associated with diving behaviour, but I found no effect of migratoriness on either Hb or Hct. Fluctuating selection was identified as the major force shaping the evolution of blood oxygen-carrying capacity. The results offer novel insights into the evolution of Hb and Hct in birds, as well as they provide a general, phylogenetically-robust support for some long-standing hypotheses in avian ecophysiology.