Final answer:
Bird and amphibian erythrocytes are nucleated, unlike mammalian red blood cells that are anucleated. The sickle shape of red blood cells is associated with sickle cell disease, which is not a feature of normal bird or amphibian erythrocytes.
Step-by-step explanation:
The erythrocytes, or red blood cells, of birds and amphibians are typically nucleated. Unlike mammalian red blood cells, which do not have a nucleus and are biconcave in shape to optimize gas exchange, the red blood cells of birds and amphibians retain their nucleus. Mammalian red blood cells are described as being metabolic anaerobes while those of other vertebrates like birds and amphibians are aerobic and capable of self-replication. When discussing abnormalities in red blood cells, it's significant to note that sickle-shaped red blood cells are a characteristic of sickle cell disease, which is a genetic condition that causes the red blood cells to become crescent-shaped and can lead to serious health issues. However, this sickle shape is not a characteristic of normal bird or amphibian erythrocytes.