From the saying: "The Early Bird Catches the Worm"
An early worm Said to a bird, "Are you the one Of whom I've heard?
"Are you the early Bird, sir, you, Who eats me if I'm early, too?"
The bird moved closer, Just a bit. Said, "I'll tell you a secret This is it:
“I'm the second earliest Bird. Today You're safe. But--tomorrow? Ah! Who can say?"
Davies plays with the old saying that the early bird catches the worm, but she tells it from the worm's side. A nervous worm, out and about early, asks a passing bird whether he's the dangerous "early bird" everyone talks about. The bird edges a little closer, which is already unsettling, and confides that he's actually the second earliest bird. The real one has presumably gone ahead and had breakfast.
The humor comes from how politely menacing it all is. The bird sounds almost friendly, sharing a "secret," yet his closing words ("Today / You're safe. But--tomorrow?") hang over the worm like a shadow. Nothing is resolved, and the worm gets only a one-day reprieve. The tone is light and sing-song, with short lines and tidy rhymes, so the mood is playful rather than frightening. A child could enjoy the joke, while an older reader might notice the quiet point about how little safety the small really have.