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 believe instead that migration has a genetic basis, with some individual birds programmed to migrate and others not.
During mild winters, more bluebirds are noticed in northern areas, but it probably just means that more have survived the milder conditions. During the coldest months, bluebirds tend to stay in very sheltered areas where they are protected from the cold winds, snow and ice. Because they don’t come out in the open as much, they aren’t as visible to us.
With the lengthening of daylight in January and February, migrating eastern bluebirds become more active and begin their northward migration. Weather has a greater influence on the northern movement than it does with the southern movement. As the mean temperature begins to remain above freezing, bluebirds begin to appear. Consequently, eastern bluebirds are usually one of the first migrants to return to their breeding areas, usually returning as far north as southern Canada by early to mid-March.
Mountain bluebirds are the most migratory of the three species. They too form into family units in late summer and merge with others to form large flocks. At this time, they may also mingle with western bluebirds. In September and early October, they depart for their wintering grounds. Northern birds start migrating sooner than southern birds. They will travel as far as southern Texas and central Mexico. The extent of their migration seems to be related to availability of fruit and severity of winter.
In milder areas, mountain bluebirds will just move to lower elevations within their breeding range. As it gets colder in the mountains, they follow the insects and berries down the mountain slopes and then back up the slopes in the spring.
During migration and winter, mountain bluebirds are found in grasslands, deserts, brushy areas, plains and lowlands. They are able to survive lower temperatures than western bluebirds, thus they are found in colder regions. In northern areas, mountain bluebirds are considered the harbingers of spring. “There is friendly competition among many bluebirders to see who can spot the first bird of the season,” says Myrna Pearman in her book Mountain Bluebird Trail Monitory Guide. “By mid-March, bluebirds have usually been observed even in the far northern reaches of their range.”
Myrna has also noted that bluebirds start arriving two to three weeks earlier in southern British Columbia than they do in southern Alberta even though they are at a similar latitude. The difference is that British Columbia is west of the Continental Divide and enjoys a milder climate than does Alberta.
Western bluebirds share some migration traits with mountain bluebirds. As stated above, in areas where their ranges overlap, they will flock together. They will also remain in their breeding range in milder areas of the west. Those that do migrate move into open scrubby forests in the foothills and canyons of the southwest.
During the winter, some of their favorite foods are the berries of junipers and mistletoe. The availability of these plants determines the birds’ movement throughout the colder weather; they can become very nomadic in their search for food. Western bluebirds consume so many berries that they are considered an important dispersal agent for the two species of plants.
All three species of bluebirds share some similar migration behaviors. They all migrate during the day and many join up with resident flocks of bluebirds to find food, water and roost sites. Fall migration seems to be determined by the shortening of daylight rather than weather. Food is still plentiful and weather conditions are still pleasant when they begin to depart. Weather can influence migration, however. Birds may linger for longer periods at foraging sites when the weather is mild. When weather turns inclement, it may cause them to move south at a faster pace.
Weather, especially the temperature, has a big influence on spring migration. As the temperature warms insects become active, and bluebirds begin their journey northward, back to their breeding territories.
Resources:
How Birds Migrate by Paul Kerlinger, PhD
Mountain Bluebird Trail Monitoring Guide by Myrna Pearman
Eastern Bluebird, Wild Bird Guides by Gary Ritchison
A Guide to Bird Behavior, Vol. III by Donald and Lillian Stokes
Cornell University Website
Ellis Bird Farm Website
Originally printed in Bluebirds Across Nebraska Newsletter BANner Volume 11 Number 2 Summer 2004