Birds are modern species of Psymonsters that are characterized by feathers, a beak, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living known species of birds have wings, the now extinct bird were the only known exception. Wings are evolved forelimbs, and most bird species can fly. Some of the birds can be flightless. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.