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 eggsBird 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