I will whistle for them: In biblical times, shepherds guided their flocks by whistling or playing pipes (see Judges 5:16).
Redeemed can also mean “ransomed,” which is buying slaves out of their servitude or indentured status (see Deuteronomy 15:15; 24:18).
Numerous as they once were: The growth of the Hebrew people was one of God's promises to Abraham (Genesis 12:1–3; 22:17; 32:12).