This video from the USA says about itself:
Brown Thrasher Molting – Bald Headed Songbird Season
26 August 2016
Late summer is bald bird or molting season. First time I’ve seen the normally aloof and well-groomed Thrasher in such a state. It will only last a few weeks.
A feather is a “dead” structure, analogous to hair or nails in humans and made of the same basic ingredient, the protein keratin. This means that when they get damaged, feathers can’t heal themselves—they have to be completely replaced. This replacement of all or some of the feathers is called molt. In addition to providing a new set of healthy feathers, molts often provide a new look to the bird’s plumage—new colors or patterns that can indicate the bird’s age, sex, or the season of the year.
Molt is extremely variable. Observed patterns can vary by species, by individual, from year to year, and by individual feathers on the same bird. Molts can be either complete, in which the bird replaces every one of its feathers over the same molt period; or partial, in which the bird replaces only some of its feathers (for example, flight feathers or body feathers).
Molt keeps birds in top flying condition by replacing feathers that have become worn or damaged with completely new feathers. However, if a bird loses an entire feather, that feather will begin growing back immediately rather than waiting for the next molt. (This is why people clip the flight feathers of captive birds rather than plucking them out).
Molting occurs in response to a mixture of hormonal changes brought about by seasonal changes. The entire process is complex and many questions remain regarding how the process is controlled. A basic understanding of molting patterns can be a useful aid in identifying many species and in determining their age.