Birds change their feathers regularly. The period, frequency and regularity of molting vary from species to species. Most adult birds molt 1 time a year in autumn, some species molt twice a year, 1 time in spring and 1 time in autumn; The female of individual bird species (such as Thunderbird) changes 3 times and the male changes 4 times in 1 year. In autumn, all the feathers are replaced, and the rest are usually replaced with some feathers. Most migratory birds migrate after molting, but some begin to migrate when molting, and some molt in wintering areas. The replacement of big feathers (such as flying feathers and tail feathers) has certain regularity. Tail feathers are generally replaced from the outermost 1 to the middle, such as pheasants; Some start from the middle, such as quail; The replacement order of woodpecker flight feathers is: the secondary flight feathers start from both sides, and the primary flight feathers start from the middle. In the molting period, birds' flying ability is not strong, and they are generally hidden. When ducks molt in autumn, their flight feathers fall off almost at the same time, and they lose their flight ability. At this time, the male duck's feather resembles the female duck, which is called boudoir feather. Most kinds of young birds begin to shed their feathers when they fly out of their nests. All birds with different colors of male and female feathers are like females; Where male and female feathers are similar, young birds are usually different from their parents. The mechanism of molting is related to endocrine function, especially when the thyroid gland is molting, the secreted thyroxine promotes metabolism and is conducive to the rapid renewal of feathers. In addition, molting and reproduction alternate, and molting is inhibited when gonads are active. Therefore, molting in autumn begins after the breeding period.