Sentence:  The dog's lean, powerful body dashed down the hill. When the dust of his violence cleared, a sea of dirty white backs was wobbling up the hill, a black-and-white quickness darting now here, now there, straightening the line, hurrying a <WSD>nibbler</WSD>, urging a straggler. 
Meaning of the word marked with <WSD> tags:  Someone who nibbles.