These are the obvious questions such observations raise. Such mystery birds are undoubtedly young brown-headed cowbirds. The cardinal feeds the young cowbird because she's been duped into raising it in her own nest. Brown-headed cowbirds are common blackbirds found on farmland, forest edges, and at feeders. Males have shiny black bodies with distinctly brown heads.
Females are uniformly grayish-brown.
The cardinal had raised the cowbird because she incubated its egg. They do not build a nest of their own. Instead, they lay their eggs in the nests of other birds. Presumably, this peculiar behavior originated on the prairies where cowbirds lived in association with the huge herds of bison that once roamed the Great Plains. The cowbirds ate insects kicked up by the bison as they grazed their way across the prairie.