What is the structural difference between the red blood cells of a sparrow and a horse?

The primary structural difference between the red blood cells of sparrows and horses is that sparrows, like other birds, possess nucleated red blood cells, meaning their red blood cells contain a nucleus. In contrast, horses, which are mammals, have enucleated red blood cells, lacking a nucleus. This structural adaptation allows birds to have a higher metabolic rate and better oxygen transport during flight. Additionally, the size and shape of the red blood cells can vary, with avian cells generally being oval and mammals' cells being biconcave discs.