The haemoglobin concentration and the total surface area of erythrocytes in the unit of blood volume decreased with similar intensity with the increase of body mass in 23 species of studied birds, weighing from 6 g to 170 g. Hence the amount of Hb per unit of the erythrocyte surface did not depend on the body mass of birds. Species of a greater body mass had smaller total surface area of erythrocytes in 1 mm³ of blood, as they had a smaller number of larger red blood cells. The changes in number and size of erythrocytes with the increase of the body mass occurred in such a way that, as a result, the value of haematocrit remained statistically the same in the studied birds.