10 



MIGEATION. 



The migration of birds, their periodical and seasonal appearance and 

 disappearance, is one of the most obvious phenomena of nature. The 

 fact that many birds disappear in winter is common knowledge and has 

 attracted attention for ages. Though once regarded as a mystery, and still 

 far from being thoroughly understood in many of its details, we are begin- 

 ning to wonder less but admire more as accurate knowledge replaces vague 

 speculation. Today, where most of our northern species spend the winter 

 is known and many of the routes by which they come and go have been 

 mapped out. We know that on the whole they are governed by ordinary 

 and well known, though perhaps highly developed, senses and common 

 every-day influences, and not by the mysterious powers and instincts once 

 ascribed to them. 



The fundamental cause of migration is obviously the waxing and the 

 waning of the food supply. Birds leave the northern land of their birth 

 because there is no other way by which to avoid starvation. Many species 

 can withstand extreme cold but none can go long without food, and though 

 some bird food still remains in Canada throughout the winter, its amount 

 is small and sufficient for only a limited population and even that supply 

 rapidly decreases, or, to the north, is buried under deep snow. The cause 

 of the southward migration in the autumn then is obvious, but why should 

 a bird leave the soft climate and plentiful food supply in the south to brave 

 dangerous travel and finally find itself in a land where retiring winter still 

 Iingtrs and the danger of starvation is imminent? Many ingenious explan- 

 ations have been advanced to account for this: longing or homesickness 

 for the land of birth, hereditary memories of an ancient home enduring 

 through geological ages, the seeking of special food for nestlings, and 

 insufficiency of nesting sites in the southern areas, have all been given as 

 possible reasons. However, it is unnecessary to advance a complicated or 

 far-fetched explanation when a simple and direct one exists. If we remem- 

 ber that in the nesting season the bird population is increased many 

 times by the birth of young; that though in winter there may be room for a 

 considerable number of birds in the southern stations, the natural spring 

 increase of population outgrows the supporting power of the land; and that 

 just at this critical time the whole northern temperate region is thrown 

 open to occupation with an abundance of food, the subject is mysterious 

 no longer. In fact, it is only by migration that it is possible to use the 

 supporting power of the temperate regions unless the birds fast or hibernate 

 through the winters, to neither of which the avian nature takes kindly. 



Though food supply is the fundamental or originating cause of migra- 

 tion we must look for other and more immediate impulses for an explanation 

 of its methods today. Originally forced to and fro by hunger, the annual 

 movements now have become instinctive and take place before the situation 

 becomes acute, the actual hunger pinch is felt, or the physical system 

 is weakened by want. 



The extent of the migrations of the different species varies. A very 

 few species do not, in the true sense of the word, migrate at all. In other 

 species only the more northern individuals recede from their stations, 

 the southern remaining practically stationary, though in the majority of 

 Canadian species the whole body moves south. The bird of greatest 



