MIGRATION loi 



when the Natural History of a sufficient number of species is 

 thoroughly worked out, we may find every link between species 

 which never leave a restricted area in which they breed and 

 live the whole year round, to those other cases in which the 

 two areas are absolutely separated." 



That migration among birds is, and has been, largely if 

 not entirely connected with the problem of food supply would 

 seem to be highly probable. A strong point in favour of such 

 an interpretation is the fact that birds which have a sufficient 

 and constant food supply do not migrate. Whole groups of 

 birds, it must be remembered, either do not migrate at all, or 

 confine their wanderings to small areas. The majority of 

 tropical and sub-tropical species, for example, do not migrate ; 

 while of typically migratory species not a few will be found 

 which have become stationary in some part of their range 

 throughout the year, having found an abundant and unfailing 

 food supply. There is no more typically migratory bird than 

 the Common Swallow {Htrundo rustica), yet a closely allied 

 species, differing only in a few slight particulars (//. savignii) 

 is resident in Egypt. The tropical and sub-tropical species to 

 which reference has just been made, are, be it noted, relatively 

 far less numerous in individuals than are such migratory species 

 as Warblers and Swallows. And this because they have be- 

 come adapted to live on a peculiar kind of diet to be met with 

 only in their own particular neighbourhood. For such species 

 migration would be suicide : thus the numerical strength of 

 such species is determined by the food supply ; a shortage 

 spells famine, and the consequent reduction of the species to 

 limits which the area can support. Migration is possible only 

 .so far from the original centre of dispersal as food is obtainable. 

 And it would seem that migratory species follow their food 

 more than is supposed. So far as is possible, overland routes 

 appear to be followed just because of the need of food all along 

 the route. "It is well known," writes Dr. Sharpe, " that in 

 certain parts of Africa, during the northern winter, vast flocks 

 of birds of prey are observed. They consist of Kites, Eagles, 

 Hobbies, Kestrels, etc., and they follow the swarms of locusts, 

 or appear in numbers where grass-fires take place." Migratory 

 birds, in short, have acquired this power to shift from place to 

 place, and the consequent advantage to the species, just because 



