Someone above said the white feather was a symbol of cowardice. I just want to expand on that - the reason that was believed is because it was though that a gamecock with a white feather in its tail was a poor fighter. However, that's only in the UK. In the US, a white feather denotes extreme bravery and excellence in combat markmanship,its most notable wearer having been USMC Gunnery Sergeant Carlos Hathcock, who was awarded the Silver Star medal for bravery during the Vietnam War.
After the First World War, when white feathers were handed out to pacifists to mark them as cowards, pacifist movements accepted the white feather as a symbol of peace - although they came up with a different reason, being the story that in 1775 Quakers in a Friends meeting house in New York were faced by a tribe of Indians on the war path. Rather than flee, the Quakers fell silent and waited. The Indian chief came into the meeting house and finding no weapons he declared the Quakers as friends. On leaving he took a white feather from his quiver and attached it to the door as a sign to leave the building unharmed.
Well, that's the historical symbolism of the white feather, anyway, but that doesn;t really answer your question.
This may, though. White has alwasy been associated with the higher planes, and with messengers of the Gods. A white feather may have a number of different meanings.
Feathers deal with ascension and spiritual evolution to a higher plane. Feathers were worn by Native American Chiefs to symbolize their communication with Spirit, and to express their celestial wisdom. Also in the Native American Indian culture, feathers represented the power of the thunder gods, along with the power of air and wind.
As a Celtic symbol meaning, the feather was worn by Druids in the form of ornate feathered robes. Celtic Druids donned these robes in ceremonies to invoke the sky gods and gain knowledge of the celestial realm. It was believed that the feathered cloak along with the presence of the sky gods would allow the Druid to transcend the earthly plane and enter the ethereal realm.
In dreams feathers mean travel or the ability to move more freely in life. White feathers in dreams indicate innocence or a fresh start in a spiritual sense.
Put simply, feathers mean:
Truth
Speed
Lightness
Flight
Ascension
Innocence