Why do reptiles and birds have a cloaca?

In reptiles and birds, the cloaca serves as a common terminal chamber and outlet for the digestive, urinary, and reproductive tracts. This anatomical structure, which is also found in amphibians and monotremes, is inherited from primitive fish ancestors.123

The cloaca functions as a centralized exit point where these distinct systems converge before emptying to the exterior of the body. In many reptiles and birds, the interior of the cloaca is partitioned by folds into specialized chambers: the coprodeum (alimentary chamber) for digestive waste, and the urodeum (urinogenital chamber) for urinary and reproductive products.12

For reproduction, the cloaca is essential for the transfer of gametes. In most birds, mating occurs via a "cloacal kiss," where the cloacal openings are pressed together to transfer sperm. Most reptiles possess an erectile copulatory organ—such as a penis or hemipenis—that is protruded through the cloaca to facilitate internal fertilization. Additionally, in terrestrial reptiles, the cloaca plays a role in water conservation by collecting nitrogenous wastes, which are eliminated alongside digestive waste.145