Senescence, or ageing, has been defined as a persistent decline in the age-specific fitness components of an organism due to internal physiological deterioration. Whole organismal declines in metabolic performance are known to occur due to ageing in avian species, as well as mammals. However, the causes for this decline in aerobic metabolism and performance are not well understood. Supply and removal of respiratory gases are key to aerobic metabolism and erythrocytes take a central role. In mammals, including humans, haemoglobin concentration, red blood cell count, and haematocrit begin to decrease in later life. In birds however, such knowledge on changes in haematological variables with increasing age is still lacking or data provide even contradictory results. We tested if haematological variables decline with increasing age in a passerine bird, the zebra finch (Taeniopygia guttata). Forty-five males aged 15 to 84 months were sampled for haematocrit, haemoglobin content, red blood cell size and number against age as a continuous factor. We found a statistically significant decline in haematocrit due to ageing in the birds (P<0.029) and a weak but not significant decline in haemoglobin, while the red blood cell size and number remained constant. The age dependence of haematological variables appears less clear in birds than in mammals. Besides oxygen carrying capacity, haematocrit influences the viscoelastic properties of blood and it remains to be shown if this decline in haematocrit in birds can explain declining aerobic metabolism.