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The shepherd hears them bleating in distress but the shepherd does not go and get them immediately. Shepherds have learned that if they go down there when the sheep first strays, the sheep will often be so foolish and skittery that they will jump right over the precipice and be killed.
The shepherds wait until the sheep are so weak that that cannot stand. Then they will go down, put a rope around them and pull the sheep to safety.
Source: From one of Gordon Macdonald's fine books.
Image: Baaaaaa!!!