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 is a nonhunted species that 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 unknown breeding or wintering areas may be prohibitive. One problem in interpretation of all banding results is the fact that recoveries often reflect the distribution of people rather than migration pathways of the birds.
Other methods used to mark individuals in migration studies include clipping the tip end off a feather (not a major flight feather) with a fingernail clipper or touching the feather with colored paint or dye. This marking technique is obviously good for only as long as the bird retains the feather (usually less than one year), but allows the investigator to recognize whether the bird has been handled previously or not.
Radio Tracking
One of the most promising methods of tracking the movements of individual birds in migration has been developed in recent years. It is called radio tracking, or telemetry, and consists of attaching to a migrating bird a small radio transmitter that gives off periodic signals or "beeps". With a radio receiving set mounted on a truck 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 Graber in 1965. He captured a grey-cheeked thrush on the University of Illinois campus and attached a 2.5-gram transmitter to it (a penny weighs 3 grams). The bird was followed successfully for over 8 hours on a course straight 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 flew 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 receivers can be mounted on orbiting satellites (Graber 1965; Bray and Corner 1972; Southern 1965).
Radar Observation
One of the developments of our modern age of electronics has been the discovery that migrating birds show up on radar screens used in monitoring aircraft. At first, the screen images caused by flying birds were a mystery to radar operators, and they designated the dots "angels." Later when their nature was understood, students of bird migration seized on the unique opportunity to obtain information on movements of birds over extensive areas (Sutter 1957; Drury 1960; Lack 1963a, b; Bellrose 1967; Graber 1968; and Gauthreaux 1972a, b).
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) a 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) a Doppler radar similar to those operated by law enforcement agencies for measuring the speed of a passing automobile. The latter radar set is useful in determining the speed of flying birds.
The use of radar in migration studies has been invaluable in determining direction of mass movement, 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 land birds migrating over the seas rather than along the coastlines and in directions observers were completely unaware of a few years ago.
Laboratory
Orientation and Navigation