174 THE STORY OF BIRD-LIFE. 



discovered, a few call-notes from the lucky finder 

 are enough to assemble the whole company to 

 share the feast. . . . One tree after another is 

 visited by the active little rovers, and its branches 

 examined : if nothing be forthcoming, away goes 

 the explorer to the next that presents itself, 

 merely giving utterance to the usual twitter that 

 serves to keep the body together. But if the 

 object of search be found, another kind of chirp 

 is emitted, and the next moment the several 

 members of the band are flitting in succession 

 to the tree, and eagerly engaged with the 

 spoil." 



This need of food, it is suggested by the same 

 writer, is probably largely responsible for the 

 movement of the birds of our northern hemi- 

 sphere, southwards, a movement which ceases 

 only at the equator. But what, he asks, induces 

 the return journey with all its attendant dangers ? 

 Probably again a lack of food, not necessarily for 

 the migrants, but for their nestling offspriog. 

 There is sufficient for the parents, but this is not 

 of a nature adapted for the requirements of the 

 nursery, which can only be found in the lands 

 lately left, where, at this time the supply will 

 meet all demands likely to be made upon it. 

 Perhaps, he suggests, this may be the explanation ; 

 but perhaps again the real motive is " the passion- 

 ate fondness for the old home." 



Indirectly, these periods of passage are a great 

 benefit to the race generally, for in the course of 

 them, the ranks of the travellers are greatly 

 thinned by reason of the numbers who fall by 

 the wayside. Only the strongest and most 



