Читать книгу Migration of Birds (1979) онлайн

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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.

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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).