Molting (from birds.cornell.edu)

Molting

A feather is a "dead" structure, somewhat analogous to hair or nails in humans. The hardness of a feather is caused by the formation of the protein keratin. Since feathers cannot heal themselves when damaged, they have to be completely replaced. The replacement of all or part of the feathers is called a molt. Molts produce feathers that match the age and sex of the bird, and sometimes the season.
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, however, be a useful aid in identifying many species and in determining their age. 

The male Indigo Bunting reaches adult plumage through a progression of molts, from the juvenile plumage (similar to the female pictured on the left), to an intermediate second year plumage and finally to the full adult plumage on the right.

There are two kinds of molts with different degrees of feather replacement.

• In a complete molt all feathers are replaced.
  
• In a partial molt only some feathers are replaced.

Damaged feathers are replaced during a molt. A feather that has been lost completely is replaced immediately.

Timing:
It takes a lot of energy to build new feathers. Molting is, therefore, often timed to coincide with periods of less strenuous demands, such as after nesting or before migration.

Molt stage nomenclature:
There are three commonly used systems for identifying different plumage stages.

The most common approach in North America is to distinguish between a winter (nonbreeding) plumage or a summer (breeding) plumage. This approach works fine on a casual basis but is inadequate in mant cases.

Molting patterns are quite variable, especially when considering tropical species or some seabirds which do not follow an annual cycle. In order to accommodate the complexity of plumages and molts the Humphrey-Parks nomenclature has been adopted by many bird watchers and scientists. (A third system is used by bird banders and is not covered here.)

Humphrey-Parks Nomenclature
This system can be a little confusing at first because in North America, many bird watchers are used to thinking of the often brighter summer plumage as a bird's main look. However, birds that have a breeding and nonbreeding plumage are usually in the nonbreeding plumage for a longer period of time, often following a full molt. This plumage is called the basic plumage.
Species that look the same way on a yearround basis are always in basic plumage.

The more colorful spring breeding plumage seen in many species is referred to as the alternate plumage under the Humphrey-Parks system.
A more complete description of this naming system, shown below, demonstrates how it is used to describe the molting and plumage patterns of a Herring Gull, which requires four years to reach adult (definitive) plumage.
Molt Cycles:
How often do birds molt? This varies by species, but almost all birds fall into one of the following three categories.
One molt per year:
Many species have one complete molt per year. These include:

Chickadees Flycatchers Hawks Humingbirds Jays

Owls Swallows Thrushes Vireos Woodpeckers

The birds shown above (left to right--Red-headed Woodpecker, Wood Thrush and Short-eared Owl) have one complete molt each year, and maintain the same appearance throughout the year.

One complete molt and one pre-nuptial partial molt:
Some species have a complete molt after nesting, molting into their basic plumage.
These species then have a prenuptial molt of body feathers that results in their bright breeding plumage.
Species with this molt pattern include:

• Buntings
• Tanagers
• Warblers

While females of these species usually look very similar on a year-round basis, they do go through a partial pre-nuptial molt and can be described as being in alternate plumage for part of the year.

The male Scarlet Tanager undergoes a complete molt in the late summer/fall that results in a greenish bird with black wings, shown on the right. In spring the male Scarlet Tanager undergoes a partial molt, including the body feathers. The new body feathers are a brilliant rich red color.
Two complete molts per year:
Only a few species undergo two full molts per year. Most of these live in areas where the environment causes significant feather wear and tear. Marsh Wrens and Bobolinks, two species that move through abrasive vegetation, are representative examples. These birds have a complete molt into basic plumage and a complete molt into alternate plumage.

These two species undergo two complete molts each year, but there the similarities end.  In the Marsh Wren (left image) both sexes have the same appearance, and that appearance is pretty much the same on a year-round basis. The two photos on the right are both Bobolinks (male and female). The male is shown in breeding plumage. After breeding all the feathers are molted and the male looks very much like the female (far right image).


Aging and a closer look at the Humphrey-Parks system:
Some species acquire their adult plumage in a single year. Others require up to five years (eagles) to reach full adult plumage. Gulls are often broken into categories such as a "three-year gull" or "four year gull," based on how long it takes the bird to reach full adult plumage.
The following chart illustrates molting and plumage nomenclature under the Humphrey-Parks system. The development of a Herring Gull, which requires four years to reach definitive (adult) plumage, is shown on the right. Note that molting periods cover a range as long as 4 months, are specific to the Herring Gull, and are approximations only. Some birds will start molting earlier and others later. All feathers are not lost at once during the molt, so there may be a high degree of variability as they molt from one plumage to the next. In Herring Gulls, the second prebasic molt may take as long as six months to complete.
The "P" in front of the molt period indicates a partial molt.

Molting patterns vary by:

• species
• individual birds of the same species
• from year-to-year
• by individual feathers

This series of photographs illustrates plumage changes in a Ring-billed Gull.

This series of photographs illustrates the molting pattern of the gorget of a Calliope Hummingbird.

References from website :  http://www.birds.cornell.edu/AllAboutBirds/studying/feathers/molting/document_view