Its long been recognized that birds and reptiles share many characteristics:
Both have skulls that abut the first neck vertebra with a single ball-and-socket joint.
Both birds and reptiles excrete nitrogenous wastes as uric acid; mammals excrete urea, fish ammonia.
Both lay similar highly yolked eggs
Bird feather is homologous to the reptile scale. Both develop from an epidermal elevation over a nourishing dermal core. Scales flatten, feather buds roll into a cylinder shaped tube