Guardian angel....

... or left behind by the bird caught by an urban fox in the garden? More info about feather symbolism here, or the sceptical amongst you would probably enjoy this comment I found in a "finding a white feather meaning" forum:

Right. So that's it, is it? That's what guardian angels do. They don't save millions from dying horrible and pointless deaths in wars, famine, tsunami and ethnic cleansing, but they can leave feathers. Wow.

Edit: best viewed large if you want to see the hidden detail....