MIGKATION OF BIRDS. 325 



in spring a comparatively small number remaining with 

 us during the winter months. 



Speaking generally, scarcity of food and unsuitability 

 of climate appear to be the chief causes of migration, and 

 it is probable that they have been the origin of the habit, 

 for birds have the means of moving rapidly across sea and 

 land from one part of the surface of the earth where the 

 conditions do not meet their requirements to another where 

 they find what they want. Thus Siberia, which, during the 

 short warm summer of that latitude, affords an ample 

 supply of food and a suitable climate to the multitude of 

 birds which migrate thither from their southern winter 

 quarters to breed, is deserted by them on the approach 

 of winter, when the land becomes deeply covered with 

 snow and all the rivers and lakes are ice-bound. With 

 the advent of summer the snow and ice disappear and the 

 birds return to find abundance of food and a congenial 

 climate for the rearing of their young. 



It is believed that in comparatively recent geological 

 times Great Britain was connected by dry land with the 

 continent of Europe, and that the birds on their passage 

 across the North Sea still follow the old coast lines of the 

 connecting ground, although these have for ages been 

 covered by the water. Geological changes usually take place 

 very slowly, and it is probable that many thousands of years 

 elapsed before the dry land which connected Britain with 

 the continent of Europe finally disappeared under the surface 

 of the sea. During the period when the land was slowly 

 sinking it may be supposed that there would be long 

 tracts of it stretching into the sea on both sides, with a 

 series of islands between them, and that the migratory 

 birds would naturally travel along the coast as far as they 

 could and then fly from island to island until they reached 

 the opposite side. When the islands finally disappeared 



