TECHNIQUES FOR STUDYING MIGRATION 



Before we discuss the many intricacies of how, when, and where 

 birds migrate, one should have a general idea of how migration data 

 are collected and what methods are currently being used to increase 

 our knowledge. Since this publication first appeared in 1935, many 

 new procedures have been used in the study of bird migration. One of 

 these, radar, has been an invaluable adaptation of a technique 

 developed for a quite different, but related, purpose. 



Direct Observation 



The oldest, simplest, and most frequently used method of studying 

 migration is by direct observation. Size, color, song, and flight of 

 different species all aid the amateur as well as the professional in 

 determining when birds are migrating. Studies begun by Wells W. 

 Cooke and his collaborators (Cooke 1888-1915) and continued by his 

 successors in the U.S. Bureau of Biological Survey (later U.S. Fish 

 and Wildlife Service) were of particular importance in the earlier 

 years of these investigations in North America. Some of the largest 

 and most interesting routes and patterns were sorted out by tediously 

 compiling and comparing literally thousands of oberservations on 

 whether a species was or was not seen in a given locality at a 

 particular time of the year. More recently, "The Changing Seasons" 

 reports by many amateur bird observers in Audubon Field Notes 

 (now American Birds) have been a most important source of 

 information on direct observation of migration. In the agregate, 

 direct observation has contributed much to our knowledge of 

 migration, but, as will be pointed out in other sections, until a few 

 years ago, observers were not aware of some of the biases in this 

 technique. 



The "moon watch" is a modification of the direct observation 

 method. It has long been known that many species of birds migrate at 

 night. Until recently, it was not apparent just how important 

 nocturnal migration really is. Significant information has been 

 derived from watching, through telescopes, the passage of migrating 

 birds across the face of a full moon. Since the actual percent of the 

 sky observed by looking through a telescope at the moon is extremely 

 small (approximately one-hundred thousandth of the observable 

 sky), the volume of birds recorded is small. On a night of heavy 

 migration, about 30 birds per hour can be seen. The fact that any 

 birds are observed at all is testimony to the tremendous numbers 

 passing overhead. Large-scale, cooperative moon-watching studies 

 have been organized and interpreted by George H. Lowery, Jr. (1951; 

 Lowery and Newman 1966). 



