Poultry flocks can pick up a variety of different parasites, both external and internal. Small infestations may not cause any health problems, but a heavy infestation can reduce performance, and animal welfare and can sometimes lead to death. It is important to monitor your flock for any signs of parasites

External

A number of different external parasites can infect a flock, typically from wild birds. Most live on the host and should be relatively easy to identify. The red chicken mite, however, does not live on the birds. Instead, they hide in crevices during the day and come out to feed on the birds at night. To check for red mites you would need to examine the birds at night.

Common external parasites of poultry (University of Kentucky)

Internal

Coccidia are single-cell protozoa that damage the lining of the small intestine. They are host-specific, meaning the species that affects cattle, swine, and poultry do not affect sheep and vice versa.

Blackhead is another single-celled parasite of poultry,  Histomonas meleagridis, which typically affects the cecal wall of the infected bird. It can be carried by chickens without doing any damage but turkeys are very susceptible.

Intestinal parasites in backyard poultry flocks (University of Florida)

Parasite management for natural and organic poultry: Blackhead in turkeys (ATTRA) - Note: The drugs mentioned for the treatment of blackhead in this publication have been removed from the market and are no longer legal for treating poultry in the USA.

Preventing blackhead disease in turkeys and game birds (Kansas State University)

Poultry and sheep:

Using multi-species grazing, including poultry, can play an important role in controlling sheep worms. Ruminants (other than sheep) serve as dead-end hosts to the sheep worm preventing the spread of larvae. Having poultry follow sheep in the rotation is another way to eliminate a lot of larvae.