Why did the Qingming Swallow come back just in time for the sacrifice?

Tomb-Sweeping Day is a traditional ancestor worship festival in China, usually from April 4th to 6th in Gregorian calendar, which is the season for swallows to return to their nests in spring. Swallows will fly to Africa or Southeast Asia for the winter when they move south in winter, and fly back to China along the East Asian monsoon belt with the help of the earth's rotation when they return north in spring. Tomb-Sweeping Day happens to be the time when swallows fly to China in spring, so in the ancient farming culture of China, people associate the return of swallows with the ancestor worship activities in Tomb-Sweeping Day.