This poem has numerous “Yeatsian rhymes,” also known as slant rhymes. These rhymes between words whose vowel sounds do not quite match were a stylistic signature of Yeats’s work. Examples of Yeatsian rhyme in this poem include “beautiful”/“pool”; “lover”/“air”; and “stones”/“swans.”
In this context, “rushes” means marsh or waterside plants that generally have stiff, hollow stems and large flat leaves. One can assume the speaker is asking where these birds will build their home; a question which signals the transience of the birds’ presence on this particular lake.
