Background for Teachers
Come Fly With Me
TPW Magazine, May 2009
Texas is one of the premiere destinations for birding in North America. Every year, hundreds of millions of birds migrate through Texas, adding to the splendor of the many resident bird species living here. About half of the 634 bird species found in Texas are migratory. Thinking of it another way, of the 338 species that mirgrate in North America (north of Mexico), 333 of them (or 98.5%) have been recorded in Texas. Texas is a haven for birds and birdwatchers, especially during migration.
Migration is more that just flying to a different area for food or a nesting spot. Migration describes movement of a population that occurs regularly and is predictable by season, path and purpose.
Migratory birds have separate breeding and wintering grounds. During the late spring and summer, these birds travel north to their breeding areas that are less crowded, have fewer predators, less competition for food and longer daylight in which to find food and feed their young. As the days shorten, weather gets colder and food supplies dwindle, these birds migrate south for the winter. Their destination "wintering grounds" provide better climate and ample food sources. Why don't they stay there to breed? Some species do live in temperate areas year-round, but other species breed more successfully with more space, less competition, longer days...so the cycle of migration continues back and forth from wintering to breeding grounds.
Migratory birds follow predicatable paths called "flyways." These flyways must provide food, water and shelter for migrating species. There are four North American flyways. Texas falls along both the Central and Mississipi Flyways. Sadly, loss of habitat along flyways is a serious conservation threat to migratory bird species.
How birds migrate is an intriguing mystery for science. For example, the ruby-throated hummingbird may fly 600 miles, 26 hours non-stop across the Gulf of Mexico and end up in the same place each year! How do they stay on course and manage such long flights?
Hummingbirds and hawks travel by day, using the sun, rivers, mountain ranges and coastlines to guide them. Songbirds migrate by night, using the stars and the earth's magnetic field. How these birds sense magnetism is still a mystery. Biologists recently discovered tiny magnetite crystals in the heads of some birds and scientists are experimenting to see how they might serve as an internal compass. Other birds sense vegetation zones and changes in humidity and temperature. Experiments are showing that birds rely on multiple factors to migrate. To learn more about migration, please try our pages on bird behavior and conservation.
Making these long journeys is dangerous. Less than half the birds that start out make it back. What behaviors help birds with their journey? Some birds wait for tailwinds and select the best altitude to help them fly. Hawks hop thermals (a column of rising air) to save energy. They fly in an upward spiral on these warm wind tunnels. Once at the top, they glide to the next thermal. Geese fly in "v" shape to reduce wind resistance on the flock, taking turns at the front of the "v" to share the burden. Songbirds depend on cooler evening temperatures to prevent overheating and cover of night to avoid some of their predators.
Before migration, metabolic changes occur in response to sun intensity and increasing day length. Food consumption increases and fat accumulates under the skin tissues. This will provide the energy for long flights. For example, hummingbirds almost double their weight with stores of fat to provide energy for their long flight. The Ruby-throated Hummingbird stores enough fat to fly 26 hours non-stop at 25 miles an hour. This is enough to span the Gulf of Mexico!
Neither metabolic changes nor any other single factor alone triggers migration. Sucessful migration is also dependent on environmental factors such as weather, arrival of spring, flowering, foliation, insect hatching and availability of food which vary from year to year. Birds can be exhausted and emaciated by the time they reach stopping areas.
Both learned and inherited traits come into play for migrating birds. Their body size and shape and the triggers that prepare them for migration are inherited, but learning their destination, food sources and evading predators are examples of learned traits.
Migrating birds must rely on good habitat along the way to rest and eat. They gorge themselves to replenish their fat reserves before preparing for the next leg of the flight. Sadly, habitat loss threatens migrating birds. Maintaining habitat, especially wetlands, for both resident and migrating birds will be critical to their survival.