In the feather pillow story, when the parasites dont have a human to feed on they usually are small. What is their usual food source and how do you know

In "The feather pillow" by Horacio Quiroga when the parasites don't have a human to feed on they usually are small, they are parasites of feathered creatures. This parasite, in particular, has been feeding on Alicia's blood, that is why it became so monstrous. We learn about this in the last parragraph of the story.