34 THE MIGRATION 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 
