Baldness in birds can have a number of reasons.
The bird uses its beak to pluck its own feathers out of its body. These can be feathers from the body itself, tail or wings. Some birds tend to mainly pluck the down feathers from the body, while others pull out the large contour feathers. What´s striking is that the feathers on the head are normal, for the simple reason that the bird can´t reach them. Plucking can have a number of causes. One of the most common reasons is probably boredom and/or stress.
Sometimes, plucking can have a physical origin. A bird could be itchy because it has skin parasites. Intestinal parasites or liver problems can also cause itchiness and make the bird start to pluck itself. If the physical symptoms are treated, it doesn't always mean the bird will stop plucking. It may continue plucking itself simply out of habit.
One treatment method is to reshape the beak so the bird can no longer grab hold of the feathers. This is, among other things, done by burning a groove in the lower beak. Another way of making sure the bird can´t reach its own feathers is by putting a collar around its neck. These are some methods to try and break the habit of feather picking. There could, however, be an underlying cause and the bird will start plucking again as soon as the beak grows back or the collar is removed.
If the physical reasons have been ruled out, it´s most likely to be a behavioural problem. Stress and boredom can be tackled in several ways. What often helps for a bird that plucks itself is to provide more distraction and to keep the environment safe and orderly.
Making the feeding session more interesting for birds is an important stimulant. A wild bird spends most of its time foraging for food and then eating it. The goal is to partly mimic what a bird does in the wild by making the feed more difficult to reach. You can find a lot of good ideas on the Internet about new and challenging ways to present the feed to your bird.
Giving your bird a partner can also solve many of the problems related to feather picking. If the bird plucks its feathers for too long, it will end up staying bald. The feather follicles are then irreparably damaged.
If feathers spontaneously fall out and no new feathers are pushed out through the skin (as they normally should), it could mean there is a skin infection that also affects the feather follicles. An infection of the skin can be caused by a fungus or by bacteria.
There are several viral diseases that can cause abnormal feather growth and feathers to fall out, such as PBFD/beak and feather disease and Polyomavirus/budgerigar fledgling disease.
General poor health
A bird’s health is important and general poor condition or wrong bird feed can have huge negative effects on the plumage. Developing new feathers use up a lot of energy and nutrients. If the bird can't get enough of these nutrients from its feed, feathers will grow back abnormally or not at all after moulting. A discolouration of the feathers can be the first visible sign of certain nutritional deficiencies.
Hormones can also influence the plumage. For instance, birds get a featherless spot on the stomach during breeding season, called a brood patch. This bald patch of skin is also well supplied with blood vessels, giving it a red appearance. This is quite normal in birds and its function is to transfer the warmth of the body from the parent to the eggs. Usually only females develop brood patches, although in some species the male can also have them.