Martins will begin their southward migration within three weeks of the young leaving the nest. The adult males are the first to leave, followed shortly by the adult females and immature birds. The southward migration normally begins in July, and most birds have gone by the end of August. Martins often gather in enormous flocks during the trip. These flocks will stay in an area for several days if the feeding conditions are good. I,m posting this for my record (Sunday -August -15 -10 Forney Texas). We had enormous flock here for about 3 weeks now none.