Earth Gauge: Just Passing Through

Birds are on the move! Migratory birds have amazing adaptations that help them find their way from their wintering grounds in Mexico, Central, and South America, to the U.S. and Canada, where they feast on abundant insects and plant foods during spring and summer. How do they know when to leave and where to go?

- Birds that fly long distances have an "internal clock" that tells them when it's time to prepare for migration and be on their way. Scientists think that subtle environmental cues (which are not well-understood in wintering grounds) trigger hormonal production in birds, which causes changes in behavior and physiology that prepare them for migration.

- Birds that migrate short distances (such as within the U.S.), including most waterfowl, learn migration routes and breeding areas from older individuals who are more experienced - usually their family members. On the other hand, most long-distance migrants are "genetically programmed" to head in a specific direction, for a specific distance. A bird's first long-distance migration is completely genetically-determined, but more experienced birds may incorporate information they've learned on the journey into their migration route. For example, if a bird finds a breeding location that is especially good, they may use learned information to return to that location in future years.

There are still many mysteries surrounding bird migration, but one thing is certain: birds need habitat and fuel to help them along their way. In the Southern U.S., scarlet tanagers, Eastern wood peewees, and yellow-billed cuckoos are passing through over the next couple weeks. A bird bath or water hole with fresh water will be welcome sight for migrants, and providing a varied supply of seeds and plants in your yard will attract a variety of feathered visitors at any time of year. Enjoy the view!

Regional migrant information was provided by the National Wildlife Federation. For more about migrants in your area, visit www.enature.com/birding/migration_home.asp.

(Source: Deinlein, M. "Neotropical Migratory Bird Basics." Smithsonian National Zoological Park, Migratory Bird Center. http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/Fact_Sheets/default.cfm?fxsht=9)