Vol. X 



IS03 



Allen, Origin and Distribution of N. A. Birds. IQ3 



bring forth their young, but are forced to retire at the approach of 

 autumn, retreating gradually before the southward advance of 

 colder weather, passing through the middle latitudes in Septem- 

 ber, and reaching the tropics in October or November, the time 

 varying more or less with different species. Here they remain 

 till the increased warmth of March or April, or the stimulus of 

 the approaching breeding season admonishes them of the necessity 

 of revisiting their breeding stations, when they begin to retrace 

 the journey toward their summer haunts, keeping pace so 

 exactly with the advance of the season as not to lose even a 

 day of the brief interval available for their sojourn in their far 

 northern home. They are thus wanderers — exiles, as it were, — 

 for three fourths of the year. Evidently our northern-breeding 

 insectivorous and berry-eating birds could not survive a winter 

 at their breeding grounds. They might perhaps be able to pass 

 the whole year in the tropics, or in the lower temperate lati- 

 tudes, — at the risk, however, of over-crowding the regular 

 occupants, and also of leaving a habitable area unoccupied. 

 As a matter of fact, nature not only 'abhors a vacuum,' using 

 the phrase in a strict sense, but allows no waste places ; living 

 space is always at a premium. Near relatives of our subarctic 

 and cold temperate insectivorous birds are found in the tropics 

 and throughout the temperate latitudes ; while the tropical forms 

 are non-migratory, those breeding in the temperate latitudes are 

 less so than their more northern kin ; they remain, owing to the 

 longer summer, for a greater length of time at their breeding 

 stations and have a shorter journey to reach their winter haunts. 



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 latitudes, 

 where can be found the food their peculiar organization renders 

 necessary. In short, as our knowledge of the habits and migra- 

 tions of birds increases, it becomes more and more evident that 

 the cause of the autumnal migration is failure of proper food at 

 the breeding station. 



Why migratory birds ever leave their winter haunts seems at 

 first sight less obvious, since in most instances it can hardly be due 

 to failure of the food supply, nor to any absolute incompatibility 

 of climate. The return in spring is often attributed to stron°- 

 home love, evidence of the existence of which is shown by the 



