SEGREGATION DURING MIGRATION 83 



adults later. It is said that in England adult cuckoos 

 depart early for the Continent, leaving their young 

 to follow as best they may. As a contrast to this in- 

 difference, we may look to the Canada goose, in 

 which families remain together through the summer, 

 adults shedding their feathers and undergoing the 

 wing moult that renders them flightless during the 

 period of growth in their offspring, so that old and 

 young emerge strong on the wing at the same time. 



Abundance of individuals of a species during 

 migration depends upon a number of factors. Kirt- 

 land's warblers are seldom seen, because they are 

 few in number and have a restricted range. Phila- 

 delphia vireos and Cape May warblers cover a wide 

 area, but are so scattered that it is unusual to see 

 more than two or three in a day under the most fa- 

 vorable circumstances. An abundance of food may 

 draw together short-eared owls, bitterns, or great 

 blue herons, species normally of solitary habit, until 

 many are assembled in a small area. This same 

 factor may assemble birds of true fleck habit in tre- 

 mendous numbers. On Great Salt Lake I have seen 

 northern phalaropes in bands of well over one hun- 

 dred thousand, drawn together by an abundance of 

 brine shrimp and alkali fly larvae. The same food 

 attractions cause assemblages of eared grebes in 

 that locality. 



In species of restricted range abundance is ob- 



