About Bird Migration
One of the most amazing phenomena, if not the most amazing of all in the wildlife kingdom, is the migration of birds. Bird migration has fascinated humans since the beginning of history. It is not only mentioned in the bible, but was studied by ancient philosophers. While many questions have been answered, many other questions remain.
Why Do Some Birds Migrate?
We know for certain birds migrate not because of cold weather itself, but because of the impact cold weather has on a bird’s food resources. If they feed strictly on insects, fruits, or rodents, chances are they will need to move to a moderate or warmer climate to find the food they need in order to survive. In respect to ground foraging birds as the Juncos who feed on seeds, they will need to relocate to an area where heavy winter snows will not cover their food resources beyond their ability to reach them for an extended period of time. So we know for certain that bird migration is driven by the availability of food. Temperatures alone won’t make most birds migrate. In fact many exotic birds could survive outside in temperatures well below zero if they had enough food.
How Do Birds Know When Migrate?
How do birds know when it is time to migrate? Well, we know that birds migrate quite punctually every year when the season is changing. And what is the most unmistakable clue to the fact that the season is changing? The length of the day! It is believed that birds can tell when the days get shorter in late summer and longer in the spring. It is believed that shorter daylight hours striking the brain through a thinning area on the top of the skull is what triggers this “alarm clock” in the fall. That combined with cooler weather and reduced food supplies tells migratory birds its time to go.
In the spring the amount of available light resets their clock. Only in this case, it’s migration northward. Certain glands in the bird begin to secrete chemicals that have to do with breeding. The bird feels the need to breed and heads north where it will be summer. So the change in the length of days and the disappearance of food tell the bird to head to warmer places. And the breeding instinct in the spring tells them to head north. There are many other factors involved, of course, and many things we still don’t understand, but these are certainly among the chief clues to bird migration.
How Do Birds Migrate?
Could you image traveling across two complete continents without a road map, and reaching your point of destination with pinpoint accuracy? The Arctic Tern, North Americas’ longest migrant, does just that. A 12,000 mile trip one way from the Arctic to the Antarctic in the fall, then returning another 12,000 miles on a different route in the spring. Or a Rose-breasted Grosbeak that nested in your yard traveling to South America in the fall, only to return to your yard the following spring. And the Dark-eyed Juncos who wintered gathering seeds under your feeder, and the same Juncos showing up the following winter in the same yard. How do birds that migrate continue to follow the same routes generation after generation for thousands of years?
Migratory birds have several methods of navigation. Birds that migrate during the day, such as hawks, eagles, and vultures, use the position of the sun to find their way. They also can use the setting sun as an indication of due west. Those which migrate at night use celestial navigation. They find their way by using the patterns of the stars. During a birds first year migration they memorize the position of the constellations in relation to the North Star. These star patterns stay the same even though the Earth moves through space, making the constellations appear to move to different spots in the sky during the year.
Birds also use magnetic fields to navigate by. They have tiny grains of a mineral called magnetite just above their nostrils. This mineral may help them to navigate using the Earth’s magnetic field, which tells the bird what direction is true north.
It is also widely believed that in their first years birds learn to identify unchanging landmarks in the landscape such as rivers, mountain ranges, and coastlines. Consider this. For at least 14 million years, Sandhill Cranes, which winter across Southern Texas, New Mexico, Arizona, and Northern Mexico, arrive by the hundreds of thousands in Nebraska on a 65 mile stretch of the Platte River. After several weeks of feeding they fan out, heading for their previous years nesting territories that stretch from Nova Scotia to Siberia.
Wingtips: Migrating flocks of birds can easily be tracked by Doppler radar, the same radar meteorologists use to track storms. Birds have a high moisture content appearing on radar as large rain drops moving in patterns not associated with weather systems. Once these radar returns are established as flocks of migrating birds, visual observation by spotters can confirm the species and approximate numbers.
How High Do Migrating Birds Fly?
The altitude at which migrating birds fly often depends on the winds, after all, it is more efficient to use wind energy. In the fall advancing cold fronts produce winds out of the north helping to propel migrating birds that are headed south for the winter. Just the opposite happens in the spring as warm winds are generated from the south. Some ducks and geese fly at remarkable heights with some geese reported flying as high as 29,000 feet, five miles up. Most songbirds that migrate at night fly between 2000 and 6000 feet, although they may fly higher to reach favorable winds.
Migration to Extinction
This is an activity related to bird migration and played by children at the Pioneers Park Nature Center in Lincoln. It is played to teach kids the hazards bird face when migrating, and they face many. Besides surviving deadly storms, bad weather, exhaustion, and other natural obstacles, migrating birds are increasingly faced with growing human threats. The destruction of habitats where birds congregate to stage their departure, or rest while on their way, reduces their ability to build up energy necessary for the long trips. This includes the draining of wetlands, forest and woodland fragmentation, and loss of native grasslands. Water pollution by man-made chemicals and non-biodegradable garbage along their routes combined with air pollution pose a huge problem. Then there are man-made objects that create lethal hazards, especially for those birds that migrate at night. Night flyers are easily misled by, or collide with, skyscrapers, wind turbines, power lines, and other unnatural structures. Then on top of all these growing perils, when they become tired, hungry, and exhausted, they easily fall prey to cats and other ground predators. And all along their journey they face the possibility of being shot by poachers, irresponsible hunters, not to be confused with those responsible hunters that follow the conservation guidelines of harvesting game birds, or those that find not just birds, but all wildlife, as expendable targets for fun and pleasure.
Wingtip: It is estimated that in the past 50 years the population of our migratory birds has been reduced by as much as 50%, mostly due to human activity.
How Do We Know Where Birds Migrate?
Bird banding had its beginning in 1902 and continues today. Birds are captured in a mist net. A small band with numbers is attached to the leg and is recorded along with other vital information about the bird. The bird is released and the data is sent to the bird banding laboratory in Washington DC. Scientists thus learn how far birds travel, how long they live, where they nest and spend the winter, whether species populations are rising or falling, and many other details of their lives. If you recover a banded bird, either dead or alive, you can report your find on line at www.reportband.gov, or by calling (800) 327-2263