3°2 
BIRDS. 
Migration. 
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 very 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. 
Distribution 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 
