30 THE MIGRATIONS OF BIRDS 



may cover several miles. This shifting is often noted 

 at the time when the young are thrown on their own 

 resources, as the parents, wearying of importunate 

 calls for food, may harry their offspring and drive 

 them from the immediate vicinity, or may accom- 

 pany them to some nearby area where food for all is 

 easily procured. In part, the wandering that comes 

 at this period may be considered merely the expres- 

 sion of a restless instinct for exploration on the part 

 of creatures endowed with wonderful freedom of 

 movement, while in part it is necessity subservient 

 to a search for food. 



The house wrens of my door-yard form ready 

 examples of the statement just made. They rear a 

 brood in a small bird-house and for several weeks 

 incessantly scan the shrubbery and herbage for food. 

 Spiders disappear from beneath porches and other 

 shelter, multitudes of aphids on the nasturtiums 

 melt away, and with tiny beetles, caterpillars, and 

 moths from grass and vines, go to feed a hungry, 

 growing family. When that family is safely on the 

 wing, the wrens may move to another locality, per- 

 haps only to a neighboring yard, perhaps farther, 

 where a less carefully explored territory is open to 

 them. At intervals the adults may return or may 

 linger to rear other broods, until the time arrives for 

 them to make their retreat to the south; but the 

 young usually disappear. This is movement in its 



