Thermoregulation constitutes an important share of the energy budget of endotherms. Elevated thermoregulatory requirements must be met by oxygen supply through the blood, as heat is produced mainly via aerobic processes. In contrast to mammal studies, it remains unclear whether elevated thermoregulatory needs are followed by changes in haematological variables in birds. We investigated haematocrit (HCT), haemoglobin content per volume of blood (HGB), number of red blood cells (RBC), and size of the erythrocytes (RBC) in zebra finches (Taeniopygia guttata) acclimated to either cold or thermoneutral ambient temperatures under laboratory conditions. Seventy-nine females were maintained for six weeks either in cold (T = + 12 °C) or thermoneutral (T = + 32 °C) ambient temperature prior to blood collection. On average, HGB, HCT and RBC were significantly lower by about 10% in cold acclimated compared to thermoneutral acclimated birds. Only RBC was not different between the two acclimation temperatures. Mean HCT, one of the most commonly measured haematological variable for example was 53 ± 0.9 % (LSM ± s.e.m) in thermoneutral and 49 ± 0.8 % (LSM ± s.e.m) in cold acclimated zebra finches. On first sight, the observed lower values for three out of the four determined haematological variables in response to acclimation to cold question oxygen supply to be indeed a limiting factor for heat production. However, higher demands of oxygen supply due to increased thermoregulation in birds may instead require specific optimisation of blood viscosity and modulation by other cardiovascular properties. Nucleated red blood cells in birds may pose different strain on blood viscosity compared to non-nucleated mammalian erythrocytes and explain the contrasting response in haematological variables to temperature acclimation between birds and mammals.