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 Even the stork in the heavens knows her times, and the turtledove, swallow, and crane keep the time of their coming, but my people know not the rules of the LORD.