Birds are vertebrates and, like mammals and other vertebrates, use kidneys as their primary excretory organs for filtering waste from the blood. The functional units within the kidneys are called nephrons. The waste product is primarily uric acid, which is then transported via ureters to the cloaca for excretion.

A. Nephridia: are the excretory organs found in many invertebrates, such as annelids (e.g., earthworms).

C. Malpighian tubules: These are the main excretory organs in most insects and some other arthropods, not birds.

D. Green glands: are the excretory organs found in crustaceans, such as prawns and crabs.