Birds are a group of warm-blooded vertebrates constituting the class Aves (/ˈeɪviːz/), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Linux (/ˈlɪnʊks/ LIN-uuks)[11] is a family of open-source Unix-like operating systems based on the Linux kernel,[12] an operating system kernel first released on September 17, 1991, by Linus Torvalds.[13][14][15] Linux is typically packaged as a Linux distribution, which includes the kernel and supporting system software and libraries, many of which are provided by the GNU Project.