By Sandy Seibert
We often think of migration as a seasonal movement of birds during spring and fall to avoid harsh weather. That is only partially correct.
“Migration evolved as a way for birds to exploit resources that are seasonably abundant and avoid times when or places where resources are scarce or weather is very harsh,” Dr. Paul Kerlinger wrote in his book How Birds Migrate.
Many birds are able to tolerate cold temperatures but if they cannot find food, they must migrate. Dr. Kerlinger goes on to write, “By far the most common type of migration, partial migration, is characterized by seasonal movements away from a breeding range by some, but not all, members of a species.”
Although each of the three species of bluebirds has their own migration habits, all three can be considered partial migrants.
By September, eastern bluebirds begin to flock. Flocks may consist of juveniles from earlier nestings or family units being made up of adults and young from the last nesting. Often, many groups will join together to form large flocks. Northern populations of eastern bluebirds will begin to move southward by the end of September or the first of October.
Eastern bluebirds do not simply shift southward. In some of the warmer areas of the country, many are year-round residents. Often, the birds from Canada and the northern U.S. will leapfrog over areas with many resident birds in order to avoid competition for food. These birds will travel as far as Louisiana, Mississippi, Florida, and the southern portions of Alabama, Georgia and Texas.
Not all northern bluebirds exhibit this type of migration. Some will migrate shorter distances and remain with resident birds throughout the winter. They will face more competition for food but, if they survive, they will have the benefit of being the first to return to their breeding area in the spring. This gives them the benefit of being able to claim the most desirable territories.
There is a certain percentage of eastern bluebirds that make no attempt to migrate south for the winter. Many researchers believe that weather has little to do with the number of non-migrating bluebirds. They