Source : Webster's Revised Unabridged Dictionary (1913)

Cowbird \Cow"bird`\ (-b[~e]rd`), n. (Zo["o]l.)
   The cow blackbird ({Molothrus ater}), an American starling.
   Like the European cuckoo, it builds no nest, but lays its
   eggs in the nests of other birds; -- so called because
   frequently associated with cattle.

Source : WordNet®

cowbird
     n : North American blackbird that follows cattle and lays eggs
         in other birds' nests