Bird Migration – Driven by Food Resources
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.
Wingtips: Other than geese and waterfowl, most birds migrate at night using celestial bodies in the dark sky and magnetic fields to find their way. At dusk and dawn they will also use terrestrial landmarks. They fly between 3000 and 5000 feet above our heads making them invisible to us on the ground. Doppler radar can track flocks of migrating birds. The moisture content of birds appear as large raindrops moving through the night sky.
A planetarium in California tested the celestial effect of the night sky on migrating birds. Using White-throated Sparrows the planetarium’s night sky showed the north star at true north and the White-throated Sparrows faced that direction, true north. After shutting off the lights they rotated the night sky 180 degrees with the north star now a southernly position. When the starry night sky was turned back on the White-throated Sparrows turned to face the north star, now to the south.
Biologists have unlocked many mysteries about bird migration, but there remains many unanswered questions. How does a bird weighing from less than an ounce to a few ounces travel from hundreds to thousands of miles locating their destination with pinpoint accuracy? It is an incredible phenomena that we should do our best to observe and celebrate.