Answer: In the story, when the parasites don't have a human to feed on and these creatures are usually quite small, their usual food source comes from feathered creatures.
Explanation:
Without human blood they are small, and they are of feathered creatures as shown in this quote: "These parasites of feathered creatures, diminutive in their habitual environment, reach enormous proportions under certain conditions." This specific parasite has been feeding off of the blood that belongs to Alicia, turning it into the horrendous way that it did, which is then learned on the final sections of the story.
