Mature granules are fusiform and many are slightly refractile. Immature heterophils have rounder granules and slightly blue cytoplasm. Avian eosinophils are slightly smaller than heterophils and their granules are round and a slightly different shade of orange. The granules of eosinophils in psittacine birds are light blue. Examples of these two types of avian eosinophils are shown here.
Avian basophils contain many small purple granules, similar to basophils of some mammals.
Avian lymphocytes and monocytes are similar to mammalian cells. Mature red cells in birds are large oval cells that contain oval nuclei. Immature red cells are rounder, have less oval to round nuclei, and polychromatophilic cytoplasm. Polychromatophilic red cells are fairly numerous in some avian species in the absence of anemia. The thrombocyte is the functional equivalent of the mammalian platelet.