SmithLifeScience

ScarletTanager.GIF (2672 bytes) Bird Migration

Last Updated     
10/12/2006     6/20/2006      3/19/2005     3/18/2005      3/17/2005     3/30/2003

Long ago, no one knew that birds migrated during the winter months. Many naturalists believed that they went underground or under the mud at the bottom of a pond to escape the cold. Aristotle, the Greek philosopher, thought that some birds changed into a different species for the winter! We now know that is not the case, but there is still a lot we don't know about bird migration. By banding some of the birds and tracking their routes, scientists have been amazed by what birds are capable of doing. As seen in the video, On a Wing and a Prayer, songbirds travel great distances, often thousands of miles. Arctic terns, for example, fly 10,000 miles from Maine to the South Pole!

Some questions remain. How do they find their way? Why do some travel at night and others during the day? How do birds instinctively know that it is time to go? More than that, how do they know where to go? There are several theories on each of these questions. Migration research has been conducted by hundreds of people throughout the years, and all of them have contributed to what we know today. Phenologists still do not know all there is to know about migration, but their studies are great examples of scientific inquiry and solving mysteries in science. In this lesson, students join the researchers to see what they can find out about the mysteries of bird migration.


W
ith regard to periodic seasonal movements, or migration, all birds can be classified as belonging to one of four groups:

  • Permanent residents, or just "residents," are non-migrating birds such as House Sparrows who remain in their home area all year round.
  • Summer residents are migratory birds such as Purple Martins who arrive in our Northern backyards in the spring, nest during the summer, and return south to wintering grounds in the fall.
  • Winter residents are migratory birds who have "come south" for the winter to our backyards. White-throated Sparrows, who are summer residents in much of Canada, are winter residents in much of the U.S.
  • Transients are migratory species who nest farther north than our neighborhoods, but who winter farther south; thus we see them only during migration, when they are "just passing through.

At whatever moment you read these words, day or night, there are birds aloft in the skies of the Western Hemisphere, migrating.

In the middle of July, Hudsonian godwits lift off from the iceberg-choked shores of the Beaufort Sea, heading southeast along the northern rim of Canada to Labrador, then vaulting south in a nonstop flight to Venezuela. 

In the snow squalls of December, goshawks and golden eagles fly south along the ridges of the Appalachians, over oak trees that rattle their last stiff, dead leaves in the wind.

Tiny song birds from Alaska leap west across the Bering Sea to the Philippines, and others from eastern Canada cross the North Atlantic to Europe and Central Africa. 

Short-tailed albatrosses from Japan glide down the coast of Washington in summer on wings as fragile as a whisper; in those same waters the albatrosses pass shearwaters from New Zealand and storm-petrels from Antarctica and the Galapagos. 
 

Recently have scientists discovered that some shorebirds apparently fly nonstop from the southern tip of South America to the coast of New Jersey, a journey of ten days—240 hours of uninterrupted flight. 

Even more remarkable are the four-ounce Arctic terns that leave the northern fringe of the continent each autumn, flying east across the Atlantic to Europe.  They push south along the bulge of Africa, recross the Atlantic to the edge of South America, and spend the winter months moving east off Antarctica.  In spring they reverse course, moving up southern Africa and lancing back to Canada'a figure eight inscribed on half the globe, a track that returns them, often as not, to precisely the same sheltered nook where they nested the summer before.

In the spring, hordes of warblers, tanagers, vireos, and other tropical migrants cross the Gulf of Mexico each night, arriving on the U.S. coast at a rate that may exceed a hundred thousand songbirds per mile of shoreline, with tens of millions making landfall each day. 
On a single autumn night several years ago, radar on Cape Cod indicated that 12 million songbirds passed overhead. 

And on the narrow coastal plane of Veracruz, Mexico, biologists discovered only recently one of the greatest raptor migrations in the world, where nearly a million hawks have been counted in a single day. In all, scientists guess,
more than 5 billion birds annually weave this incredible tapestry across the hemisphere. 

Although migratory behavior is inherited, birds do not migrate without the proper hormonal stimuli. The pituitary and adrenal glands play an important role in stimulating a migratory bird to prepare for migration and to initiate migration. Just before fall migration, birds accumulate a thick layer of fat just under the skin, a response triggered by hormones. Birds can gain from 3-4 percent of their body weight each day. For a 200 pound human, this would be like gaining 6-8 pounds per day. For example, a Blackpoll Warbler weighing 11-12 grams will double its weight to 20-23 grams just before migration. This fat provides enough energy for the Blackpoll Warbler to fly nonstop for 85 hours over the Atlantic Ocean to its South American winter home. Once the fat layers are in place, cold weather will often initiate the actual departure from the summer range.

There are four flyways in North America. Review them with the class by pointing out the routes on the map. The Atlantic Flyway goes from Florida up the Atlantic coastline, then stretches from the Northeast over to the Great Lakes. The Mississippi Flyway goes from Louisiana up the Mississippi River then stretches from the Great Lakes west to the Dakotas (Chicago is in the Mississippi Flyway). The Central Flyway goes from Texas and New Mexico up to Montana. The Pacific Flyway goes from California up the Pacific coastline to Washington. Encourage students to list these flyways in the space provided below their own maps. Each group should determine which flyway was used by the migrant they have plotted, and record it at the bottom of the data sheet.

Teacher Key:
Atlantic Flyway: prothonotary warbler
Mississippi Flyway: green heron
Central Flyway rose-breasted grosbeak and black-whiskered vireo
Pacific Flyway: tree swallow and black-billed cuckoo 

 

Complete
 There are complete migration patterns, when all members of a species leave the breeding range. In this pattern, there is no overlap between where they spend the winter and where they spend the summer. the migration pattern of the black-whiskered vireo, the rose-breasted grosbeak, and the prothonotary warbler, for example, is complete in this lesson.

Partial
 There are partial migration patterns, when some, but not all, of the member sof a species travel from the breeding range. this is the most common pattern. Robins migrate from regions with harsh winters, but in milder parts of therir range like the Puget Sound, they stay all winter long. In this lesson, the migration patterns of the green heron and the tree swallow are partial.

Irruptive
 There are irruptive migration patterns, when migrations are not as predictable. These flexible migrants are more like food specialists that travel where they need to depending upon the conditions of that particular year. In some years, red crossbills migrate south, but they do not do so every year. This lesson shows the black-billed cuckoo's migration pattern as irruptive.

Do birds fly with the sun as a cue?

Key ideas:
Birds may use the sun as a cue while they are flying. For example, if flying north, they may know that the sun should be kept on their right in the morning and on their left in the afternoon. Birds may also calibrate their own direction senses to other cues like stars or magnetic compasses by noting where the sun is setting. The plane of polarized light caused by the setting sun could be a very reliable cue. This idea would account for both diurnal and nocturnal birds.

Examples of past research:
Frank Moore of the University of Southern Mississippi studied whether birds use the sun as an orientation cue. Using Savannah sparrows he found that the accuracy of orientation was best when the setting sun was visible. When the setting sun was blocked by covers or clouds, this accuracy was reduced significantly. He placed mirrors around their cages to alter the position of sunset. When sunset was shifted 90 degrees to the true sunset position, the birds shifted their orientation 90 degrees in the same direction. Without the sun, or the polarized light it produces, the birds lost their sense of direction.

Some things to think about:
Navigation by the sun is not as simple as it seems, however, because you must know the time of day fairly accurately. Also, what happens on cloudy days? Although some birds do migrate during the day, the majority do so at night. Sun navigation cannot account for over 90% of migration which takes place at night.

Do birds fly with the stars as a cue?

Key ideas:
When birds fly at night, they may use the stars to find their way. Caged birds who see the stars in a planetarium show migratory restlessness and often face the direction they should be flying. Many birds migrate at night, and may use the stars as their guide.

Examples of past research:
A German scientist used European warblers, some of which had never seen a real sky, to show that birds do pay attention to the stars. When the planetarium sky was matched to the real sky on a particular night, the birds inside were oriented in the same direction their wild relatives were flying outside. When the planetarium sky was changed to match a sky hundreds of miles to the east, the birds oriented in such a way as to get back on the right course.

Some things to think about:
This investigator used very few birds and other researchers have not been able to replicate his results. Also, what happens on overcast nights when the birds cannot see the sky?

Do birds fly with the earth's magnetic field to guide them?

Key ideas:
The magnetic field is a force surrounding the earth. Scientists think that magnetism is the most important directional cue used by migrating birds. Birds may use the built-in compasses in their bodies to find the poles. The magnetic force gets stronger as they get toward the poles. Even on cloudy days, birds could use this method.

Examples of past research:
Scientists have tied small magnets to the wings of pigeons and found that they homed just as well as control birds carrying an equal weight of non-magnetic metal. The earth's magnetic field did not seem to help them, but more research is needed.

Some things to think about:
Birds are capable of using several cues to orient during migration, including the moon, the sun, stars, wind, magnetism, topography, and olfactory cues. With so many possibilities, it is exceedingly difficult to study one cue in isolation from others.

How do the birds know that it is time to start migrating?

Key ideas:
Birds may be able to tell that it is time to go by using changes in amount of light, temperature, or food. As winter comes, for example, the daylight hours are reduced and the temperature goes down. These cause the amount of food to change, too.

Examples of past research:
Scientists once thought that birds knew to migrate in the spring because it got warmer in the spring, but that was not reliable enough because some springs were cooler than others. Finally they concluded that it was the increase in the length of day in as spring advanced. It has also been concluded that males leave the tropics earlier than females so they arrive about one to four days earlier. Competition for food and nesting sites would be in favor of males more than for females.

Some things to think about:
It is important to recognize which are direct causes and which are indirect. When food is needed the most, it becomes very scarce: insects die, water freezes, rodents hibernate, and birds leave. The lack of food may very well be the direct cause for the birds to migrate, but the light and temperature may be indirect causes.

How does weather affect bird migration?

Key ideas:
A migrating bird doesn't rely on sight alone. Their vision at night is not even as good as ours. Birds flies with the air mass. The fact that they migrate in summer and fall has less to do directly with temperature and more to do with the fact that air patterns are changing. They do not see well, so they have to trust that the north or south wind will take them the right course. Sometimes things go wrong.

Examples of past research:
Frontal movements are correlated with large numbers of migration birds. Whenever a south wind switches to west on nights when birds are migrating, a drift of dead birds on the beaches of the Atlantic coast is common. On April 16, 1960, this kind of tragedy happened on the shores of Lake Michigan. A migration flight was taking place on the south winds along the west shore when the wind abruptly changed direction and started blowing from the west. The birds were blown out over the lake on winds reaching 80 miles per hour. A squall with hail then beat them down into the water. On the next morning, dead birds were found along 35 miles of Indiana Shoreline. Counts covering 25% of the dunes indicate that a total number of birds who died may have been 12,000. There were at least 56 species involved. The wild migrants are what pilots call "pressure pattern" flyers. This simply means that they only fly if the air mass is going their way on south winds in spring and north winds in fall.

Some things to think about:
Not all birds fly with the wind. Swallows and swifts, day migrants who feed on insects in the air as they fly, migrate against the wind.