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; 

 Lacke 1963a, b; Bellrose 1967; Graber 1968; and Gauthreaux 1972a, 

 b). 



Three types of radar have been used for studing 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 __ 



Studies on how migrating birds orient (travel in one compass 

 direction) or navigate (travel toward a specific goal) have received 

 increasing emphasis in the past 20 years. These studies have focused 

 on the ability of birds to orient themselves by the position of the sun 

 and stars. Outstanding in this facet of research have been the works 

 of Matthews (1951, 1955), Kramer (1952, 1959, and 1961), Sauer and 

 Sauer (1960), Mewaldt and Rose (1960), Sauer (1961), Hamilton 

 (1962a, b), Schmidt-Koenig (1963, 1964), and Emlen (1969). The basic 

 method used in the experiments is to observe the direction in which 

 confined birds attempt to move during the period of migratory 

 restlessness. The birds are not permitted to have any view of the 

 landscape but only the sky above them. In some cases the positions of 

 the celestial bodies are changed by the use of mirrors to see the effect 

 on the orientation of the experimental birds. In other cases the 

 experiments are performed in plantetariums so positions of the stars 

 in the artificial heavens can be manipulated and the effect observed. 



Physiology of Migration 



The physiological basis for bird migration has received 

 considerable attention, particularly the effects of seasonal increases 

 and decreases in daylight and the seasonal rhythms occurring within 

 animals and referred to as "biological clocks." Investigations in this 

 field include the pioneering work on the relationship of photoperiod 



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