We know, for instance, that Arctic terns migrate from the Arctic to the Antarctic and back each year. We know that ruby-throated hummingbirds winter in Costa Rica. We know that American goldfinches don't migrate in the intercontinental sense but wander widely in search of food throughout the winter. We know that cardinals stay put year-round.
While migration itself boggles the mind, how do experts know when, where, and which birds migrate?
Certain tools help scientists understand birds' sometimes long-distance travels:
Second, existing large datasets archive long-term information on annual, seasonal, monthly, and daily migrations. These datasets include tallies from bird counts, completed by birdwatchers on the second Saturday of May and again in late December. The more than 100 years of data are adjusted statistically to account for variables like weather, birdwatcher skill, and the number of folks counting. Over the long haul, results are telling.
The datasets have swollen in recent years, moving well beyond the twice-a-year counts. Citizen science online reporting sites such as eBird (www.ebird.org) compile birders' sightings into time-series migration maps. Any of us can check maps on any day for any location and any species to see what's where at the moment.
Third, acoustic monitoring of migratory flight calls, especially at night, identifies migrating species. Originally, acoustic monitoring was only as good as the listeners' ears and identification skills. Now, digital recordings, enhanced and subjected to computerized analysis, identify species and individual numbers.
Fourth, radar, now among the most helpful of the newer technology tools, detects and tracks migrating birds, sometimes even identifying large masses by species. On your home computer, you can choose the date and watch, in one-hour increments, the nightly mass of migrating birds. Go to www.badbirdz.com/fall2004index.htm. It's an amazing sight!
Fifth, in a more advanced scientific analysis, stable isotopes measured in feather samples identify by longitude, latitude, and altitude the geographic location in which those feathers grew. Knowing where a bird molts and grows new feathers unravels many mysteries.
Last but probably most exciting, radio-telemetry devices like geolocators can trace birds' daily movements, making fascinating study. Certain backpack-like devices require recapture of the "wired" bird, so that device is used only on birds known for their loyalty to ancestral sites. Two years ago, for instance, scientists first learned where black swifts wintered--all because one keen-eyed birder spotted the back-pack-toting bird upon its return to Colorado--from Brazil!.
While experts are far from knowing all they want to know, they're learning more every day. Consider contributing to the data base by joining the twice yearly counts and also reporting your observations--even those in your backyard--on the eBird site.