I04 A HISTORY OF BIRDS 



reason their descendants continue in the same unshaken faith. 

 In other words, our summer migrants are so many British 

 races of their species, though indistinguishable from their con- 

 tinental relatives. Now for the sequel to this. As, by human 

 interference — drainage, agriculture, persecution at the hands of 

 game-preservers and collectors — these British races become 

 reduced, their numbers are not replaced. The Bittern, Avocet, 

 Ruff, Spoon-bill, Harriers, Ospreys, and the dozen or so more 

 of our vanishing native avifauna that turn up year by year 

 among us are the last survivors of these old races. When they 

 are gone their places will be taken only by occasional, accidental 

 stragglers blown out of their course by adverse winds. Their 

 place will not be taken by annual visitants because the repre- 

 sentatives of these species on the mainland of Europe have 

 similarly become " local " races^ — -using this term in the wide 

 sense — which therefore return year by year to their accustomed 

 breeding spots, and as their ranks in turn become depleted so 

 will they too become rare, then vanish. 



The intimate relation between migration and food supply 

 is demonstrated annually within the confines of the British 

 Islands during such visitations of " hard " weather as occur 

 during the winter months : and on this head both Barrington 

 and Eagle Clarke, our foremost authorities, have furnished some 

 remarkable evidence. 



Though we must still regard the origin and survival of the 

 migratory habit as a problem never likely to be certainly solved, 

 we may yet find some satisfaction in hypothesis. 



Thus then it is to be noticed that migratory species, 

 individually, vastly outnumber sedentary species, and this 

 because of the enormous breeding areas which this wandering 

 habit secures. But migration is only possible where a sufficient 

 supply of food is to be secured, both on the journey and at its 

 terminus — and this is true even where great distances can be 

 covered without food. Very well. We may assume that the 

 migratory species owe their origin to the matter of food supply. 

 Composed of individuals subsisting on a food of universal 

 range, but limited in supply, they were enabled to roam farther 

 afield as their numbers strained this supply in their immediate 

 neighbourhood. Annually, however, a check was placed on 

 further extensions of range by the cares of breeding and by 



