Codex on the Flight of Birds of a kite as seeking a wind current. When the winds are high, one will see the bird very high in the sky, but when the winds are low, the bird stays closer to the ground. Leonardo describes how a bird rests in the air, after flapping its wings to gain altitude, by gliding downward to the ground. Folio 6 contains multiple diagrams of birds flying and their bone structure, and all of the commentary is on flight and how birds fly the way that they do. Leonardo starts off by describing how a bird ascends or descends in different
Psittacine beak and feather disease and beak make tissues vulnerable to secondary infection. Because the virus also affects the thymus and Bursa of Fabricius, slowing lymphocyte production, immunosuppression occurs and the bird becomes more vulnerable to secondary infections. Beak fractures and necrosis of the hard palate can prevent the bird from eating. The ornithologist Edwin Ashby observed a flock of completely featherless red-rumped parrots ("Psephotus haematonotus") in the Adelaide Hills, South Australia, in 1888. The species then disappeared from the area for several years. The condition is more prevalent in widely occurring Australian species such as the sulphur-crested cockatoo, little corella and galah. The first