Before The Angel Experiment
The Flock was created in the School as a genetic experiment.
The Angel Experiment
A few avian-human hybrids were released from the Institute for Higher Living in The Angel Experiment (such as Bird-Girl).
School's Out - Forever
Max II was introduced.
The Final Warning
Total, the Flock's dog, began to develop wings.
Maximum Ride Forever
Harry was first introduced.
All avian-human hybrids have six limbs, tend to weigh a bit less than a hundred pounds, need between 3000 - 4000 calories a day, have air sacs in addition to lungs, and some other qualities of birds. In addition, they also have their own directional senses/navigation skills, raptor vision, acute hearing, and other advanced senses.
In School's Out - Forever, Max said that the Flock—like birds—have nuclei in their red blood cells.
Harry and his flock are a variation of avian-human hybrid, being more birdlike and having feathers covering their bodies.
It is possible for a "bird kid", as they are sometimes called, to mutate on his/her own after a few years, as pointed out by Jeb Batchelder in The Final Warning. This was proven true in the Flock's case, as all six original members developed at least one rare or super-human ability at some point in the series.