Birds are commonly infected by gastrointestinal worms. There are a number of intestinal parasites that affect birds and have different symptoms and treatment.
Common parasites of pet birds
Pet birds can get roundworms from wild bird droppings that fall into their cages. Affected birds are weak and suffer from profound weight loss. Without proper intervention, death can occur. This is an important reason why routine deworming of pet birds should be undertaken.
Giardiasis infection is most common among lovebirds, cockatiels, and budgies; it can also affect larger species of parrots. Transmission occurs thru direct contact with contaminated material or from an adult bird that is carrying the protozoa, Giardia spp. Affected birds suffer from diarrhea and malnutrition. Feather plucking and itching have been observed in cockatiels, causing the bird to vocalize loudly and pick at its skin with its beak. Baby birds may die before fledgling stage if medication is not administered.
Tapeworms are found in African greys, cockatoos, and finches. The worms can easily transfer between birds via an intermediate host (earthworms, spiders, insects, etc.) that have picked the tapeworm from the droppings of infected birds. Fortunately, there is medication that your vet can provide to kill the tapeworms.