Cavity nests, such as those excavated by woodpeckers, are perhaps the safest nest sites. Such nests are easy for humans and predators to find but are difficult to enter. Only a few snakes, long-armed raccoons, and flying squirrels can plunder these nests. Starlings can even drive woodpeckers away from newly excavated holes. The scarcity of such cavities explains why most cavity nesters adapt so readily to nest boxes.
Some species change nest sites as the season progresses.
Cardinals and rufous-sided towhees, for example, nest on the ground early in the season in April when there are no leaves on the shrubs. After leafout they switch to shrubs. Other species such as grackles and doves nest in the foliage of evergreens in March and April and switch to broadleafed shrubs after leaves appear. Indigo buntings and field sparrows start nesting in woody shrubs when they leaf out and switch to large weeds later in the summer when they have grown enough to support nests. Nests constructed of fine fibers located in thistles are likely to be those of goldfinches, which often nest and feed in thesame thistles.