AVES 



583 



This is traversed fall and spring and largely over water. The aver- 

 age migrating bird travels only about 23 miles a day on migration, 

 but this tern makes at least 150 miles a day. Recently the U. S. Army 

 Signal Corps reported that one of its messenger pigeons had flown 600 



Fig. 305. — Migration airways of birds in North America. (From Metcalf, Textbook 

 of Economic Zoology, published by Lea and Febiger.) 



miles in less than 14 hours (42 miles per hour) without the aid of a 

 tail wind. Several theories attempting to explain bird migration in- 

 volve such factors as food conditions, temperature, changing length 

 of days, and hormone control. The explanation is still an open 

 question. 



