They're in a sub-clade. Related to reptiles, but not reptiles themselves. Birds are warmblooded, reptiles aren't.
A) Bone histology of archosaurs suggests that archosaurs were ancestrally warmblooded and modern archosaurs like crododiles are secondarily warmblooded due to their ecological niche.
B) Organisms can't outgrow their ancestry and are always modified versions of their ancestors so if birds are sub-clade of reptiles that makes them reptiles.