They're not. All birds are bipedal and are descended from bipedal dinosaurs that existed long before bipedal mammals. Those dinosaurs and their bird descendants - which are technically still dinosaurs by the way - are both classified as "theropods" due to their hip structure, which is what enabled them to become bipedal in the first place, as opposed to the "ornithischian", mostly quadropedal dinosaurs which are completely extinct.

In dinosaurs and birds, bipedalism evolved as adjustments to the hips and leg orientation. In humans, the spine also underwent significant changes to support a fully upright posture.