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Direct Observation

More recently, the National Audubon Society and many state Audubon and ornithological societies publish information in their bulletins and newsletters on direct observation of migration. In the aggregate, direct observation has contributed much to our knowledge of migration, but this method is limited by its being largely restricted to daytime, ground-based data on birds either before or after a period of actual migratory flight.

The "moon watch" is a modification of the direct observation method. Many species of birds migrate at night. Until mid-century, it was not apparent just how prevalent nocturnal migration really was. Significant information has been derived from watching the passage of migrating birds across the face of a full moon through telescopes, noting both the numbers and directions of flight. Since the actual percent of the sky observed by looking through a telescope at the moon is extremely small (approximately one-hundred thousandth of the observable sky), the volume of birds recorded is small. On a night of heavy migration, about 30 birds per hour can be seen. The fact that any birds are observed at all is testimony to the tremendous numbers passing overhead. A large-scale, cooperative moon-watching study was organized and interpreted by George H. Lowery, Jr. of Louisiana State University in the 1960's.

Aural

Preserved Specimens

Marking

The records of banded birds have also yielded other important information relative to migrations, such as arrival and departure dates, the length of time different birds pause on their migratory journeys to feed and rest, the relation between weather conditions and starting times for migration, the rates of travel for individual birds, and the degree of regularity with which individual birds return to the summer or winter quarters used in former years. Many banding stations are operated systematically throughout the year and supply much information concerning the movements of migratory birds that heretofore could only be surmised. The most informative banding studies are those that focus on particular populations of birds. Examples of such planned banding programs are the extensive marking of specific populations of ducks and geese on their breeding grounds by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service, as well as "Operation Recovery," the cooperative program of banding small land birds along the Atlantic Coast. When these banded birds are recovered, information concerning movements and survival rates of specific populations or the vulnerability to hunting is gained. Colored leg bands, neck collars, or streamers can be used to identify populations or specific individuals, and birds marked with easily observed tags can be studied without having to kill or recapture individuals, thus making it a particularly useful technique.

We have learned about the migratory habits of some species through banding, but the method does have shortcomings. To study the migration of a particular species through banding, the banded bird must be encountered again at some later date. If the species is hunted, such as ducks or geese, the number of returns per 100 birds banded is considerably greater than if one must rely on a bird being retrapped or found dead. For example, in Mallards banded throughout North America the average number of bands returned the first year is about 12 percent. In most species that are not hunted, less than 1 percent of the bands are ever seen again.

In 1935, Lincoln commented that with enough banding some of the winter ranges and migration routes of more poorly understood species would become better known. A case in point is the Chimney Swift, a common bird in the eastern United States. This species winters in South America. Over 500,000 Chimney Swifts have been banded, but only 21 have been recovered outside the United States (13 from Peru, 1 from Haiti, and the rest from Mexico). The conclusion is simply this: whereas banding is very useful for securing certain information, the volume of birds that need to be banded to obtain a meaningful number of recoveries for determining migratory pathways or breeding or wintering areas may be prohibitive. One problem in interpretation of many banding results is the fact that recoveries may often reflect the distribution of people rather than the distribution of birds.

Radio Tracking

Radar Observation

Three types of radar have been used for studying birds: 1) general surveillance radar, similar to ones located at airports, that scans a large area and indicates the general time and direction of broad movements of birds; 2) tracking radar that records the path of an airplane (or bird) across the sky by "locking on" to a designated "target" and continuously following only that object; and 3) Doppler radar similar to those operated by law enforcement agencies for measuring the speed of a passing automobile or by meteorologists for detecting tornadic winds. The data collected by radar can be electronically stored in the absence of a human observer and can be correlated with weather data sets.

The use of radar in migration studies has been invaluable in determining direction and speed of mass bird movements, dates and times of departure, height of travel, and general volume, especially at night. One interesting fact to come out of current radar work is the discovery of relatively large movements of warblers and other small land birds migrating over oceans rather than along coastlines and in directions about which ground-based observers were completely unaware.