Disease - birds do not normally loose their feathers. When they change them it is called moult.
Moulting is a process (like shedding in mammals) done regularly by birds, to replace old feathers with new, healthy feathers. This is normal. When a bird loses many feathers at once, it is usually due to the bird plucking out its own feathers OR a condition called French Moult, for which there is no cure.
All birds may or may not lose their feathers. It is only when this happens to them and or they die.
Feathers
Thunderbird
All of them.
Bird feathers.
Your bird could be suffering any number of ailments, from a cage mate who is plucking it, to French moult. If you have ruled out that another bird is plucking the feathers, take it to an avian veterinarian to help uncover the cause.
The kiwi bird is a unique bird that has hair-like feathers on its body.
Down comes from young chicks.
how the feathers protect the bird
As far as we know, there is no bird living without feathers.
When a bird molts, it looses some feathers and gets new ones. Usually in the spring, but at other times of the year also.
how the feathers protect the bird
It will not be able to fly with no feathers. Feathers keep a bird warm. If its feathers are gone it will die of cold. Most importantly feathers help a bird to escape from predators.