The process of molting is when a bird will loose their feathers a few at a time. This will occur symmetrically or one from each side in order not to affect flight. The length of the entire process various from either weeks or months depending on the time of year it is occurring and the species of bird. This process would not be noticeable to the casual birdwatcher.
Hummingbirds will begin to molt between September to November. It will then take several weeks for each feather to regain its size and shape.
Why is molting necessary? Molting allows the bird to replace their worn out feathers with new ones. Feathers are vitally important to the bird. The feathers help provide weather protection for the bird and in some bird species (not hummingbirds) the molting process can even help the birds attract a mate.