What is Migration?

Migration is movement. It is a process. It is an observable phenomenon for those willing to see it, and is almost always evocative when witnessed.

Migration applies to animals as well as plants. It occurs worldwide to populations as well as to species within a microclimate.

The very existence of migration drives scientists to study it as surely as it inspires artists to render it and poets to capture its essence in words.

There are many different definitions of migration, but all include these essential qualities: migration exhibits predictable movement of an animal from one location and climate to another location and climate. Typically these movements are linked to resource availability, seasonal changes and reproduction.

Some of birds migrate during the day. These diurnal (day) migrants include shorebirds, hawks, ducks and geese, as well as some songbirds. But the bulk of the songbirds migrate at night.

Nocturnal migrants such as warblers, vireos, thrushes, and sparrows leave their daytime habitats just after dusk and spend the next eight to ten hours flying. Near dawn they descend to another site along their migratory route, and spend the day or next few days feeding and resting until they can continue the journey.

Each species employs its own strategy for migration, but all require two important things during their migration—food and rest. Maine habitats provide both of these necessities. Habitats or natural communities that consistently provide migrants with the opportunity to refuel and rest during their journey are known as stopover sites. If the area provides this service to a large number of migrants, the stopover site is considered to be significant and its significance is determined locally, regionally, hemispherically or even globally. Stopover areas may not always be apparent to the casual observer, as they may not concentrate extraordinary numbers of the larger, more visible migrants. When birds arrive in an exhausted state with little or no fuel reserve, they will feed quickly to strengthen themselves and renew their reserve before moving on to better habitats to accomplish the job of full refueling.

DIURNAL VS. NOCTURNAL MIGRATION

The benefits of diurnal migration exhibit themselves clearly in the largest group of daytime migrants, the hawks. These migrants depend heavily on riding rising air currents called thermals. As these air currents heat up during the day and rise in the atmosphere, birds take advantage of this lift. Many species thermal hop—they rise to the top of one thermal, set their wings in the direction they want to travel, coast to the next thermal, and then repeat the process.

The simple fact is that thermals do not occur at night in such a quantity to allow these birds to utilize them successfully. Other thermal riding or soaring birds include storks, pelicans and cranes. Some neotropical songbird species like kingbirds, swallows and blackbirds will migrate during the day, but the vast majority are nocturnal migrants.

As a rule most birds (excluding owls, night-herons, goatsuckers and some other species) are typically diurnal during most of the year, but they migrate only at night.

Nocturnal migrants tend to be birds that have long distances to fly and do so in powered flight. At night the atmospheric structure is much more stable. It is cooler and smoother than during the day. The coolness helps birds to maintain healthy body temperature without large water losses, while the smoothness of the air allows for a straight level course without expending energy correcting and maintaining a course in turbulent air. The cover of night is also a great way to avoid predation.

GETTING FROM POINT "A" TO POINT "B"

Scientists who study bird migration generally agree that there are three primary ways birds find their way between point "A" to point "B."

Piloting is probably the most simple. It involves following a series of familiar landmarks to a predetermined destination. Without knowing it, we pilot to and from work, the supermarket or school by using familiar landmarks like streets, signs or buildings. For birds flying several thousand feet above the ground, landmarks are often on a grander scale, like mountains, rivers or coastlines.

Birds, like airplanes, follow the coastline. Rivers also direct birds and fish north and south. Even the Maine Turnpike may be a landmark for a migrating bird—this long, gray, north/south running road could viewed as a river! However, landmarks are not always visual, so their use is limited to those animals that migrate during the day.

Orientation is the second method birds use to find their way. During the last 50 years, scientists have discovered that migrating birds use the sun, the stars and the earth's magnetic field as compasses for orientation. Because these features vary little from year to year, they are reliable directional cues.

True navigation, the last mechanism of direction finding, requires that an animal determine its position relative to the position of its destination and then travels a course to reach it. This is especially important when birds are blown off course by strong winds or during bad weather. Birds displaced during migration are likely to encounter unfamiliar landmarks. Orienting on a fixed heading relative to the position of an external cue may result in a bird flying farther off course.

How birds accomplish true navigation is still a matter of conjecture. One possibility is that birds possess an internal map. With its ability to orient based on using external cues like star patterns, its internal clock and the map, a bird could plot its position and steer a course home. Another possibility is that birds create mental maps as they travel the migration route. If displaced or disoriented, they could retrace their flight path and get back on course.

Unfortunately, little evidence is available to support either hypothesis of true navigation in birds.

With thanks to our friends at New Jersey Audubon Society