It's Jeremiah, Chapter 12, Verse 9 (Jer 12:9):Mine heritage is unto me as a speckled bird, the birds round about are against her; come ye, assemble all the beasts of the field, come to devour.Click here for the entire chapter, so you can read it in context.
The Revised Standard Version translation is:Is my heritage to me like a speckled bird of prey? Are the birds of prey against her round about? Go, assemble all the wild beasts; bring them to devour.I think the RSV is a little easier to understand.
But in the song, the church is like the great speckled bird.