We've looked up the bird in a field guide. It is a male scarlet tanager. The greenish looking bird is probably a female. Scarlet tanagers can be found in Illinois. Some are just migrating through Illinois on their way north.
Tanagers feed on insects and fruit.
Tanagers are interesting to us, because they spend the winter in tropical rain forests and then migrate to North American woodlands for the summer. In winter they can be found in Panama and Columbia, south of the Andes to northwest Bolivia. This habitat also provides important resting areas during migration. Habitat destruction is one of the leading causes for declines for all types of wildlife. Preservation must include the habitat in our own backyard, here in the United States and here in Illinois.