svanstippen
Svanstippen is a colloquial term, primarily used in Sweden, that refers to the very tip of a swan's tail feathers. This specific part of the plumage is often white, even on swans that have darker body feathers. The term itself is a direct translation of "swan tip" from Swedish, with "svan" meaning swan and "tippen" meaning the tip.
While not a formal ornithological term, "svanstippen" is commonly used to describe this distinctive white marking.