On a Wing and a Prayer
Migration Mysteries Clue Cards
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.