A crow/rook sits up on your ariel.
What Does It Mean To Find A Black Feather Outside Our Home?
A bird of some sort with black feathers has been by there.
Some kinda black bird got scared and lost the feather.
Maybe a cat or another animal got a'hold of a black bird and ate it.
A bird with black plumage has lost a feather and it has just ended up outside your house.