What does Preacher: Must Feed The People (2) mean in the Bible?

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Charles Spurgeon's Illustration Collection - Preacher: Must Feed the People (2)
Everybody knows that large flocks of pigeons assemble at the stroke of the great clock in the square of St. Mark: believe me, it is not the music of the bell which attracts them, they can hear that every hour. They come, Mr. Preacher, for food, and no mere sound will long collect them. This is a hint for filling your meeting-house; it must be done not merely by that fine, bell-like voice of yours, but by all the neighbourhood's being assured that spiritual food is to be had when you open your mouth. Barley for pigeons, good sir; and the gospel for men and women. Try it in earnest and you cannot fail; you will soon be saying, 'Who are these that fly as a cloud, and as doves to their windows?'