Migratory birds refer to those birds that have migratory behavior. They travel back and forth between their breeding grounds and cold refuges along fixed routes every spring and autumn. Qiao Anchen is such a "migratory bird". Every summer she "flies" across the ocean to return to her nest. Two months later, when the return time comes, she has to cross the ocean to "overwinter" in a foreign country.
In contrast, Ke Yihang is a "resident bird" who lives in one place all year round and leads a stable life. However, he has been waiting, waiting for the "migratory bird" he loves to stop for him and guard their "bird's nest" with him.