<p>This study provides an annual overview of erythrocyte cytomorphometry and physiology in a passerine finch, the Scaly-breasted Munia (<i>Lonchura punctulata</i>). Birds were procured from nature and maintained in an outdoor aviary. Blood samples were collected from five adult male birds per month for one year, and erythrocyte cytomorphometry was done for 30 erythrocytes per bird. In total, 1,800 erythrocytes were measured. Simultaneously, we recorded their seasonal physiological parameters, namely body mass, surface body temperature, and testicular growth and recrudescence cycle along with relative humidity and ambient temperature. Further, we tested rhythmicity in the seasonal profile of all the parameters. We observed that the erythrocytes and their centrally situated nucleus varied from circular to elliptical in shape. The dimensions of erythrocytes changed differently in different season, e.g. erythrocytes were smaller and relatively circular in shape during summer and larger and elliptical during winter season. The erythrocyte elongation ratio was minimum by the end of monsoon season but nucleus/cytoplasmic ratio was minimum during the beginning of the monsoon season. Testicular volume and surface body temperature showed a rhythmic seasonal pattern; however, body mass remained unchanged and statistically arrhythmic. The results suggest that seasonal changes in the environment affect the erythrocyte cytomorphometry and physiology of Scaly-breasted Munia. Since erythrocytes are at the centre of oxidation–reduction dynamics, these data could be useful in veterinary science to compare the health status of other resident passerine finches of similar body size and to compare whether the test animal is following a normal seasonal rhythm or not.</p>