Final answer:
Pterosaurs are not considered bird ancestors because they lack feathers, which are a defining characteristic of birds that evolved from theropod dinosaurs.
Step-by-step explanation:
Pterosaurs and pterodactyls are ruled out as the ancestors of birds because they lost an important avian feature, the feather, long before birds appeared. Birds are believed to have evolved from theropod dinosaurs around 150 million years ago, with feathered dinosaurs like Archaeopteryx and Xiaotingia serving as evidence of this evolutionary link. Unlike birds, pterosaurs had wings formed by membranes of skin, attached to an elongated fourth finger, and lacked feathers. Furthermore, birds exhibit a number of unique characteristics related to flight that pterosaurs did not have, such as an elongate S-shaped neck and a specialized uropygial gland essential to preening, which helps to waterproof and maintain the flexibility of their feathers.