146 PROCEEDINGS OF THE ACADEMY OF [April, 



Waves and Their Components. 



Accepting the fact that the migratory movement advances by 

 ."waves" or "rushes," — that is to say that the bulk of the migration 

 at each locality occurs on certain nights or series of nights, — ^the 

 question naturally arises: To what extent are the several ''waves" in 

 successive years composed of the same species? 



A study of the migration curves will show that there are from eleven 

 to fourteen prominent waves during the Spring, taking into considera- 

 tion only those which show ten or more arrivals* in Februar}^ and 

 March, fifteen to twent}'- in April, and thirty to one hundred in May. 

 These seem to me to be the only movements worthy to be styled waves, 

 although some have used the term to indicate far less marked move- 

 ments, while others use it only for the most extensive migrator}^ flights.-'^ 



Selecting forty-seven common species for which we have the fullest 

 data, and noting such migrator}^ activity'' as is indicated by each on 

 .the wave-days for the years 1904 to 1907, we find a remarkable corre- 

 spondence in the species which make up each wave. And the same 

 ''wave" may be recognized through a number of years by its com- 

 ponent species, though its date may vary considerably. Sometimes a 

 movement may be interrupted by unsuitable weather and be resumed 

 again later, making two apparent waves in one year which correspond 

 to one in other years. Or when conditions are exceptionally favorable 

 early in the season, the species which usually compose Wave II, for 

 instance, may push forward and form part of Wave I; and although 

 conditions at the normal time of occurrence of Wave II may be favor- 

 able there will be no movement, simply because all the species usually 

 migrating at that time have passed on. 



It seems then that certain species migrate together, advance strag- 

 glers of some accompanying the bulk movements of others, and that 

 each species is ready for migration at approximately the same time each 

 year., the exact date depending upon a favorable combination of 

 meteorological conditions. 



The following tables will show which of the forty-seven selected 

 species composed the various waves for the four years for which Ave 



'''Arrival" here has the same significance as explained on page 193. 



^ Cf. Twenty-five Years of Bird Migration at Ann Arbor, Micliigan, by N. A. 

 Wood, Eighth Anmial Report Mich. Acad. Sci. 



" Usually only the "first arrival" -nithin the Pliiladelphia circle and the one 

 or more marked bulk movements are considered, but sometimes when the first 

 arrival was a very early straggler the second arrival is also noted. 



