274 AUTUMN AND WINTER 



on migration in great numbers. But the waste of life from 

 storm on passage or untimely and exceptional cold on arrival 

 is probably more than made up by the advantage of rearing 

 their broods in the most favourable circumstances. 



Just as all existing species of birds are the result of a 

 continuous chain of evolution, of which many of the connect- 

 ing links have not been preserved, so the great movements 

 of the migrants between their summer and winter homes are 

 probably the outcome of a gradual and tentative process of 

 migration, which has been fixed on its present lines by the 

 survival of the fittest. Most birds live in flocks for the 

 greater part of the year, only separating for the compara- 

 tively short nesting-season ; so that gregariousness seems their 

 natural and earliest habit. As they broke up for the breed- 

 ing-season, and at first spread evenly outwards from their 

 gregarious haunts, they would come sharply into competition 

 with other birds expanding outwards in the same way. 

 Gradually the struggle for life would settle which group of 

 species was the strongest in each region ; those species which 

 were best adapted to its peculiar conditions would prevail, 

 and those which were less well adapted would tend to die 

 out within this area, but would have a better chance to the 

 north, where fresh lands lay open each spring. Their 

 migrations would further tend to be controlled by their 

 power to endure the winter climate of their new homes. If 

 they could pick up a living there in winter as well as in 

 summer, they became resident species ; if not, they became 

 what we call summer migrants. Winter visitors — a term 

 which is used more often than winter migrants, but precisely 

 corresponds to it — are the summer migrants of more northern 

 regions, viewed from the winter end of their journey. Such 

 are the redwings and fieldfares, which usually arrive with us 

 in November ; these are summer visitors to Norway and 



