Final answer:
The presence of nucleated red blood cells in avian and reptile blood makes it more difficult to accurately determine WBC counts, as opposed to mammalian blood where red blood cells lack nuclei.
Step-by-step explanation:
The accurate determination of avian and reptile white blood cell (WBC) counts is more difficult than WBC counts in mammalian blood primarily because of the presence of nucleated red blood cells in avian and reptile blood. In mammals, red blood cells lack nuclei at maturity, which makes it easier to differentiate them from WBCs during laboratory counting. However, in birds and reptiles, red blood cells retain their nuclei, which can complicate the distinction between red and white blood cells under a microscope, leading to possible miscounting or misidentification.