Capillaria contorta (Eucoleus contorta), are a type of hairworm or threadworm that are common internal parasites of domestic poultry and wild birds. They are very thin, filamentous worms measuring 15-25 mm long (males) and 35-80 mm long (females). C. contorta are found most frequently in the bird's mouth, esophagus and crop.
C. contorta has an indirect life cycle, and produces eggs that can infect birds 4-6 weeks following passing in feces. Birds are infected by consumption of earthworms that are infected with the eggs.
Description of environment where they are most likely to be found
C. contorta are often found in outside environments where birds are kept in a confined area with poor-drainage and ventilation. Birds are often fed off of the ground and have access to earthworms in the soil.