Jeremiah points out the behaviour of migratory birds, such as the stork and the swallow. They know exactly when to start the migration to the south and when to fly back. This is their nature. But the people of God do not return to him. This is unnatural and shows unfaithfulness to God and his laws.
7 Même la cigogne connaît dans les cieux sa saison; La tourterelle, l'hirondelle et la grue Observent le temps de leur arrivée; Mais mon peuple ne connaît pas la loi de l'Eternel.