INTERNAL MIGRATIONS 277 



migration and local movement, I am compelled to 

 believe that it is governed by law. 



In the British Islands these local migrations apparently 

 are only undertaken in winter. I say apparently, because 

 it is by no means proved that there is not a considerable 

 amount of local movement taking place at other seasons 

 and entirely overlooked. In winter birds are more 

 gregarious, more easily observed, and the initiating 

 causes of local migration, such as rises and falls in 

 temperature and storms, are readily and easily defined. 

 In spring and summer much of this local movement 

 might go on, as indeed I strongly suspect it does, with- 

 out being noticed. Birds both in winter and summer 

 follow their food, the general area of their distribution 

 is that in which they can find sustenance, and as this 

 food-supply varies a good deal according to season, 

 both in the matter of description and locality, it is only 

 natural that birds should undertake some local journeys 

 in quest of it. So far as I can ascertain, all this internal 

 migration and local nwuement takes place witJiin the 

 normal area of dispersal of every species. There is no 

 evidence whatever to show that a species will wander 

 from its normal area in quest of food, and the laws 

 which govern its dispersal inexorably confine it to that 

 area, and the species will perish therein if the conditions 

 change and render existence impossible. As I have 

 previously shown, species never extend the area of their 

 dispersal in winter, emigration or range expansion can 

 only take place in summer or just prior to the season of 

 reproduction. It must also be remarked, that not a 

 single species taking part in these local movements in 

 winter in our area is non-indigenous to that area. No 



