Migration of Birds 307 



Thus at a single stretch they travel about 2400 miles, 

 remaining on the wing for probably 48 hours. The 

 energy required for this prolonged flight is attained by 

 consuming the fat stored in their bodies. They start 

 fat- and become lean during the journey. If a 1000- 

 pound airplane could be propelled with as little waste 

 of energy in proportion to its weight, i gallon of 

 gasoline would suffice for a flight of 160 miles. 



Classification according to migratory habits. With 

 reference to the time a species remains in a given place, 

 birds are divided into four classes : (i) permanent resi- 

 dents, (2) summer .residents, (3) winter residents, 

 (4) transient visitors. The last class includes those 

 which occur only during migration. 



If the birds that may be seen in the course of a year 

 near your home were classified in this way, which of the 

 four classes do you think would include the largest 

 number of species and which the smallest number? 

 About how many birds would be in all four classes ? To 

 which class do most of the warblers belong ? You might 

 make four lists of the birds that occur in your neighbor- 

 hood, classifying them in this way. 



Oh, happy life, to soar and sway 



Above the life by mortals led, 

 Singing the merry months away, 



Master, not slave of daily bread, 

 And when Autumn comes, to flee 



Wherever sunshine beckons thee ! 



LOWELL 



