Black magpies fly south, literally meaning crows and magpies fly south. The deeper meaning may be that under the pressure of Cao Cao's army, many people rushed to the southeast (Sun Quan's side). The next sentence is "no branch to follow", which means there is no safe place.
Cao Cao may have written this sentence to persuade those who fled to Jiangdong to turn back quickly. Taking refuge in Sun Quan has no good result.