The stork... knows the time of her migration... But not my people! They do not know the Lord's laws.
Twitchers are very keen bird-watchers. They will flock to any place where they can get a sighting of a strange-looking bird which may have got blown off its migratory course. Jeremiah certainly knew the habits of the storks, the turtledoves, the swallows and the cranes. These birds all flew over Jeremiah's land on their migration to the north and then to the south again. They knew just when it was time to do this.
Bird-watching was one thing, but watching how God's people in Jeremiah's time were living showed a very sad picture. They stubbornly "ran down the path of sin" (verse 6). Our verse at the top of the page says, "they do not know the requirements of the Lord." (verse 7, NIV) Of course they could tell you what God's laws were, but they had no intimate concern for them — they stubbornly turned their backs on God. They listened to false prophets instead.
When people do that, they end up disastrously. Sin is a very poor paymaster. "The wages of sin is death..." (Romans 6:23). Jeremiah warned the people about this; he pleaded, he wept and still they ignored him as he spoke God's call to sinners who kept running away from God. They didn't know what they were missing — the free gift of God's eternal life and the wonder of his grace and love.
Dear Lord, I too have a stubborn heart. Turn me to yourself and to your ways. Amen.