Читать книгу Migration of Birds (1979) - Frederick Charles Lincoln - Страница 8

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An adjunct to the previously described nocturnal observation methods, which has potential for species identification, is the use of a parabolic reflector with attached microphone to amplify call (chip) notes (Ball 1952; Graber and Cochrane 1959). 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 and there is considerable difficulty in identifying the chip notes made by night migrants. In addition, the bird may not call when it is directly over the reflector and consequently it would not be recorded. These calls are quite different from the notes we hear given by familiar birds during the daytime while they are scolding an intruder or advertising their territory.