Blue jay nests consist mostly of twigs, robin nests of mud and twigs, and wood thrush nests are lined with soft, partially decayed leaves. These are the easiest nests to find in winter. A few smaller species also nest on wide branches. Pewees and cerulean warblers build small, shallow nests on the tops of branches and cover the outside with lichens and bark. These nests are tough to spot in any season.
Pewees guard their nests with aggressive defensive behavior.
One of the characteristic sounds of Illinois woodlands in the summer is the snapping of pewee bills as they mob blue jays and brood-parasitic cowbirds, which lay their eggs in the nests of other species. Many species nest in dense shrubs near the ground. Such shrubs can support large nests but are highly vulnerable to predators. Shrub nesters may need every month of the spring and summer to fledge just one brood successfully. Fox and rat snakes appear to be the major predators of shrub nests in Illinois.