302 BIRDS. 



Very little is, however, at present understood with regard to the signification of 

 egg-coloration. Frequently the different species of a group lay very similarly 

 coloured eggs, as is exemplified by the warblers and buntings ; but this is by no 

 means invariably the case, as is well shown by the different members of the thrush 

 family. In many cases the coloration of the eggs is evidently adapted to the hue 

 of their natural surroundings, as is well exemplified by sandpipers, dunlins, 

 plovers, and their allies, and likewise by pheasants and partridges. 



Since no bird hibernates, while a large number breed in regions 

 where they could not possibly exist during the cold winter months, 

 it is essential that they should migrate to warmer regions in which to pass that 

 season of the year. Such migrations may be vgry partial, as is the case with 

 many British species, when the individuals passing the summer in the more 

 northern parts of the country come further south during the winter ; while those 

 from the area into which the immigrants arrive likewise move southwards. From 

 such partial migrations there is a gradual transition to complete migrations, when 

 the birds of one country travel to a far distant land for the winter. As the great 

 masses of land enjoying a cold climate are mainly confined to the Northern 

 Hemisphere, it is obvious that bird migrations must take place from south to 

 north, and the following general laws of migration are now accepted. With the 

 exception of purely tropical species, every bird breeds in the coldest or most 

 northern part of its range ; such nesting-grounds being generally reached by 

 a horizontal migration, although in a few instances birds may ascend mountains 

 until they meet with the required degree of temperature. This northerly 

 migration is always for the purpose of breeding, while the southward return is 

 for food and warmth. Those species which go furthest north often also range 

 furthest to the south ; while every species has its particular period of migration. 

 Finally, no species ever breeds during its sojourn in the southern portion of its 

 migratory area. 



It would be quite out of place to enter into any discussion as to the origin of 

 this migratory instinct ; but it may be mentioned that as the young frequently 

 make the autumn migration unattended by the old, it is quite evident that the 

 journey is made independently of any knowledge of the route. Moreover, as most 

 migrations take place in the night, it is clear that this alone will preclude any 

 guidance of the host by landmarks. Then, again, from the circumstance that 

 during astronomical observations flights of birds have been seen crossing the 

 moon's disc at an immense elevation above the earth, there is good reason to 

 believe that at least many migrations take place at heights whence the con- 

 figuration of the continents and oceans would be invisible even during the day. 

 Nevertheless, it appears that there are certain definite lines along which vast 

 numbers of birds, subject to conditions of weather, habitually migrate ; one of 

 these trunk-routes passing through the island of Heligoland and along the 

 western coast of Europe. 



. Although, from their power of flight and migratory habits, it 



might seem that Birds would have no definite distributional areas, yet 



this is by no means the case ; and the different zoological regions into which the 



world is now mapped out were originally defined from the various groups of Birds 



