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Migration of Birds

Techniques for Studying Migration


Since this publication first appeared in 1935, traditional methods as well as new procedures have been used in the study of bird migration. On occasion a method developed for a quite different but related purpose has become an invaluable innovative technique in our continuing exploration of the migration phenomenon.

Direct Observation

The oldest, simplest, and most frequently used method of studying migration is by direct observation. Size, color, song, and flight of different species all aid the amateur as well as the professional in determining when birds are migrating. Studies by Wells W. Cooke and his collaborators from 1888 to 1915 and continued by his successors in the U.S. Bureau of Biological Survey (later U.S. Fish and Wildlife Service) were of particular importance in the earlier years of these investigations in North America. Some of the largest and most interesting routes and patterns were sorted out by tediously compiling and comparing literally thousands of observations of species in a given locality at a particular time of the year.

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

Another nocturnal observation method which has potential for species identification during the study of migration is the use of a parabolic reflector with attached microphone to amplify call (chip) notes. This device, when equipped with a tape recorder, can record night migrants up to 11,000 feet on nights with or without a full moon. A primary disadvantage is that one cannot tell the direction a bird is traveling. Furthermore, there may be some difficulty in identifying the chip notes made by night migrants, since these calls are often different from the notes heard during the daytime. Unfortunately, the bird may not call when it is directly over the reflector and consequently it would not be recorded.

Preserved Specimens

Reference material consisting of preserved bird skins with data on time and place of collection exists in many natural history museums. The essential ingredient in studying migration by this method is to have an adequate series of specimens taken during the breeding season so differences in appearance between geographically separated breeding populations of the same species can be discerned. Such properly identified breeding specimens may be used for comparison with individuals collected during migration to associate them with their breeding areas. This provides a convenient way of recognizing and referring to individuals representative of known populations wherever they may be encountered.

Marking

If birds can be captured, marked, and released unharmed, a great deal of information can be learned about their movements. Many different marking methods have been developed to identify particular individuals when they are observed or recaptured at a later date. Since 1920, the marking of birds with numbered leg bands in North America has been under the direction of the U.S. Fish and Wildlife Service (and more recently the Biological Resources Division of the U.S. Geological Survey) in cooperation with the Canadian Wildlife Service. Every year professional biologists and volunteers, working under permit, place bands on thousands of birds, both game and nongame, large and small, migratory and nonmigratory. Each band carries a serial number on the outside and an address where recovered bands can be sent on the inside. When a banded bird is reported from a second locality, a definite fact relative to its movements becomes known. The study of many such cases leads to a more complete knowledge of the details of migration.

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

Radio tracking, or telemetry, is accomplished by attaching a small radio transmitter that gives off periodic signals or "beeps" from a migrating bird. With a radio receiving set mounted on a vehicle or airplane, it is possible to follow these radio signals and trace the progress of the migrating bird. One of the most dramatic examples of this technique was reported by Richard Graber in 1965. He captured a Gray-cheeked Thrush on the University of Illinois campus and attached a 2.5-gram transmitter (a penny weighs 3 grams). The bird was followed successfully for over 8 hours on a course straight north from Urbana, across Chicago, and up Lake Michigan on a continuous flight of nearly 400 miles at an average speed of 50 mph (there was a 27 mph tail wind aiding the bird). It is interesting to note that while the little thrush flew up the middle of Lake Michigan, the pursuing aircraft skirted the edge of the lake and terminated tracking at the northern end after running low on fuel while the bird continued to fly on. The limitations of radio telemetry, of course, are the size of the transmitter that can be placed on birds without interfering with flight and the ability of the receiving vehicle to keep close enough to the flying bird to detect the signals. Despite this difficulty, there has been considerable development in the technology, and encouraging results to date give promise for the future, particularly when birds can be tracked by orbiting satellites. Yet this technique should be used cautiously, since several studies have demonstrated that transmitter-equipped birds have significantly lower survival.

Radar Observation

Radar was developed to identify and track aircraft electronically and was an innovation that was critical to England's success in the Battle of Britain during the early years of the Second World War. Early radar observers noted, however, that they received moving returns that could not be associated with aircraft. These radar echos, whimsically termed "angels" by observers in England, were soon discovered to be birds. That bird flight could be monitored by radar was seized upon by students of migration after the end of the war as an opportunity to obtain information on the movements of birds during both day and night and over extensive geographic areas.

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.


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