The Collaborative International Dictionary of English v.0.48:

Cowbird \Cow"bird`\ (-b[~e]rd`), n. (Zool.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cowbird
    n 1: North American blackbird that follows cattle and lays eggs
         in other birds' nests