Bird-Kids are human-avian hybrids.They are 98% human and 2% bird. They have wings, air sacks, hollow bones and they are stronger, faster and have more stamina that normal humans. They also have to consume more food than normal humans in order to survive. Bird-Kids were made by the school. Their genes have been programed to keep evolving so they can develop special skills such as breathing under water and reading minds.
Maximum Ride, Fang, Iggy, Nudge, The Gasman (Gazzy), and Angel are all Bird-kids. Maya (Max's clone), and assumedly the other two clones (one of Nudge, one of Angel), and Dylan are also Bird-Kids, despite being clones of other people.