The normal modern meaning: a swan of the genus Cygnus. Written 白鳥 in general writing and often ハクチョウ in species-name contexts.
In winter, swans come to this lake.
The children fed the swans from the shore.
white bird
Literal use based on the characters 白 and 鳥, referring to a white-colored bird in general. For an ordinary description of any non-swan white bird, 白い鳥 is often clearer.
Taken literally, 白鳥 can also refer to a white bird.
Every winter, swans migrate to this lake.
A flock of swans was floating on the lake.
A swan is gracefully floating on the quiet lake.