1 -JO Jones, Bird Migyafion in lotva. YhprW 



to be assigned to any group. Hence, a given species may be 

 found in either one of two or more waves in successive years. 

 The species which always migrate in company, but at irregular 

 times in successive years, are more likely to be found in the same 

 relative wave each year than any others. Phoebe came with the 

 third wave each year except one. Meadowlark, assigned to no 

 group, came with the first wave once, the second twice, and the 

 third once. 



The term *' bird wave " has been freely used. What is a bird 

 wave ? In his excellent ' Report on Bird Migration in the 

 Mississippi Valley,' on page 26, Prof. W. W. Cooke thus defines 

 it : — 



"(i) A ' bird wave ' may be considered to consist of a very 

 large number of individuals of one or many species, which sud- 

 denly invade a certain area. . . . 



" (2) Certain species known to be migrating in company on a 

 given day may be considered to constitute a ' wave.' " 



Hence, a ' bird wave ' consists of not only the species arriv'mg 

 at a certain time, but also all which are found to be increasing or 

 decreasing in numbers, or departing northward. All of these 

 movements are represented on the charts. There are other 

 movements of scarcely less importance in the migrations which 

 cannot be so represented. Such are the movements of individual 

 birds of resident, winter visitor, transient and summer resident 

 species, without any apparent increase or decrease of the numbers 

 in a given locality. It is evident that such belong to the ' bird 

 wave,' even though the charts take no account of them. Hence, 

 a bird wave is often, if not always, much larger than can be 

 shown by any graphical representation. This unrepresented 

 movement has been of great service in determining the bird 

 waves of apparently small extent. In the larger movements its 

 value is less evident, but it has a no less active part. 



Taking all sorts of movements together, there are two periods 

 of maximum bird activity during the migrations. The first one 

 occurs during the middle of March, when the first really spring 

 warmth sweeps over the country. The second occurs during the 

 first half of May. This one is finely shown on the charts. During 

 this period there is a massing of species which is almost bewil- 



