It often seems that every time a woodpecker drills a nesting cavity, a pair of bully European starlings comes along and drives the woodpeckers away. Time and time again, I have watched a pair of red-bellied woodpeckers, northern flickers or red-headed woodpeckers, spend days drilling a hole in a tree suitable for nesting and raising young. Just about the time the cavity is ready for eggs, a pair of starlings will take over the cavity and keep the woodpeckers out.
Even worse, after the starlings have won the battle, which usually takes a couple of days, they will often occupy the cavity for only a few more days, and then move on to the next conquest. By the time they vacate, the woodpeckers have drilled another nesting cavity, and the vicious cycle begins again. The same scenario may occur with bluebirds trying to settle into a birdhouse.
European starlings were introduced into North American in 1890, supposedly to control Japanese beetles, but the birds, themselves, have become a greater nuisance, impacting on the breeding success of many native birds.
Along with blackbirds, European starlings gather in large flocks for winter, and generally move south. But come March, the starlings fly back to their breeding grounds, pair off, and begin the quest for a nesting cavity.
Because European starlings are not protected by laws as are native birds, they can be discouraged in a number of creative ways to keep them from bullying native birds that are trying to raise families.
-- George H. Harrison