34 THE MIGRA TION OF BIRDS. 



space is always at a premium. Near kin of the Warblers, 

 Swallows and Flycatchers which visit the subarctic and cold 

 temperate districts are found in the tropics and throughout 

 the warmer temperate latitudes ; while the tropical forms are 

 non-migratory, those breeding in the temperate latitudes are 

 less so than their more northern relatives ; they remain, 

 owing to the longer summer, a much longer period at their 

 breeding stations and have a shorter journey to reach their 

 winter quarters. 



This may serve as a general illustration, showing that the 

 absence of proper food in the high north forces the summer 

 insectivorous birds to leave these regions for warmer lati- 

 tudes, where a perpetual summer insures at all times the 

 supply of food their peculiar organization renders necessary. 

 In short, as our knowledge of the habits and migrations of 

 birds increases it becomes more and more evident that the 

 cause of the autumnal migration is failure of proper food at 

 the bird's breeding stations, the breeding habitat being also 

 the bird's true home. Elsewhere it is an exile and a 

 wanderer, most of the interval between the breeding seasons 

 being spent in leisurely journeying to and from its breeding 

 station and its winter haunts. 



In many instances birds remain at comparatively northern 

 localities during the winter if, through some unusual cir- 

 cumstance, they find their favorite food abundant, even if the 

 weather prove more than usually severe. This is often 

 illustrated in the case of the Robin and several species of 

 Warblers and Sparrows. Yet, the fact remains that, in the 

 case of a great many insectivorous species which breed in 

 the high north, failure to migrate would bring certain de- 

 struction, due to decrease of temperature. This is shown by 

 the fact that not unfrequently many birds are destroyed on 

 their northward migration in spring by encountering too 

 great a fall of temperature in late, unseasonably severe spring 

 storms. 

 • Why migratory birds ever leave their winter quarters is 



