The baby swallows can gather flowers and return home after their first flight.
The meaning of "swallow's first flight" is that the newly grown swallow begins to learn to fly. It is often used to describe someone doing something for the first time. Returning from picking flowers describes gaining something after learning to fly, echoing the story of the baby swallow's first flight.
The upper and lower sentences of the four-character plum blossom poem have four characters. Like an idiom, the verses must be neatly contrasted and catchy.