both birds and mammals are warm-blooded (homoiothermous), meaning they can maintain a constant body temperature . They also both breathe using lungs . Regarding their skeletal structure, birds possess a fully ossified (bony) endoskeleton , a trait shared by mammals as vertebrates whose embryonic notochord is replaced by a bony vertebral column . However, viviparity is not a shared characteristic; birds are oviparous (they lay eggs), while mammals are mostly viviparous (giving birth to live young), with rare exceptions such as the Platypus .