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Radar Tracking of Experimentally Released Migrant Birds

1978; Wiley; Volume: 49; Issue: 4 Linguagem: Inglês

10.2307/4512393

2327-1280

Natalie J. Demong, Stephen T. Emlen,

Noise Effects and Management

During the past two decades, much attention has been directed toward understanding the navigational systems used by migrating birds. Surveillance and radar tracking studies of bird migration have provided descriptive information on the behavior of birds in flight. Experimental studies on captive animals have investigated the types of sensory cues that animals can use, at least in caged situations. These studies indicate the probable existence of a hierarchy of redundant, multiple cues available for orientation and navigation. Rather than search for the mechanism of orientation, one must perform experiments that will separate various component cues and provide insights concerning their relative weighting and importance for orientation (Emlen, 1975). In this report we outline a new technique that allows one to manipulate experimentally the navigational system of a free-flying bird as it travels along its migratory route. In our studies we use a lightweight cardboard box, suspended beneath a helium balloon, to carry an individual passerine migrant aloft to migratory altitudes. At a prescribed height above ground, the trap-door floor of the box falls open and the bird flies out. As the bird selects its departure direction and continues its migratory flight, it is observed or tracked from the ground. It thus becomes possible to exercise a degree of manipulative control over the orientational information available to a free-flying migrant in a way not previously possible. For example, one can release migrants under a variety of naturally occurring meteorological conditions, including some when birds normally choose not to go aloft. One can deprive birds from viewing local landmarks, sunset cues, or other celestial sources of information. And one can experimentally alter various aspects of the orientational system. By observing and comparing departure orientations under different conditions, it becomes possible to study the relative importance of, and the interactions between, the different directional information sources used by migrants aloft.