Miles seems to have in mind the Brown-headed Cowbird (Molothrus ater) which is native to most of North America and breeds in the southern United States. Our Southern Birds explains:

This is the only bird we have in America who neither makes a nest nor cares for its own young. The female lays her white egg and then, watching her chance, slyly carries it to the nest of some smaller bird when the home-builders are absent, and leaves it to the care of more honest and responsible parties. Warblers, Sparrows, and Vireos are all victimized in this manner; and while some Warblers are bright enough to outwit the imposition by building a new nest on top of the first, they will not do so if their own eggs are already placed, but take upon themselves the extra task of hatching and rearing the young Cowbird.   (113-4)

Miles goes on to say that the young Cowbird, larger and more demanding than the "rightful nestlings," often receives more food at their expense. She allows her contempt for the bird to overcome her scientific objectivity as she provides the following explanation of the bird's name: "They are oftenest seen walking about singly or in promiscuous groups among cows in pasture" (114). Ornithologists now explain the species' unusual breeding habits in terms of their parasitic relationship with roving herds of livestock; cowbirds feed on insects that follow bison, cattle, etc., and thus cannot maintain stationary nests of their own.