The four calling birds represent the four books of the Gospel, calling us to come to Christ. Each one sings the same song (salvation in Christ) but the pitch is different, sung to different ears…
Matthew — calling the Jews to their Messiah.
Mark — calling the Romans to the one true God.
Luke — calling the Greeks to the Savior of humanity.
John — calling the world to its Maker.
No matter which book sings its way into your heart the best, the tune is the same: Jesus is calling you to come to Him and be saved.