MIGRATION OF BIRDS. 153 
so commonly around the upper waters of the Clyde and 
Tweed during the vole plague ; and at present, widgeons and 
scoters are showing an inclination, in small numbers, to halt 
and breed with us. In other places where the natural 
conditions have remained the same yet, some birds have 
moved on—eg., when cattle or sheep have been allowed to 
disturb their haunts. 
As for the routes that are taken, these mark the direction 
of expansion from the original centres: all birds do not 
_know the way back to these centres from experience, as 
in many cases the young birds lead the way. What we 
might call the sense of direction is a hereditary possession 
of birds, not there originally, but has, through an ordinary 
law of Nature, become a part of their being. It is implicit 
obedience to this law, unconscious obedience, that prevents 
birds from losing their way or following other familiar bird- 
routes. Wheatears and white wagtails from Iceland will 
ever continue to reach the Continent by the west coast of 
Scotland. Fieldfare, redwing, ortolan, and Richard’s pipits 
may travel together till they reach the south coast of 
Norway; but thereafter, while the first two species will 
come over to our country in vast thousands, the ortolan 
and pipit will still bend southwards over Heligoland, down 
the coast of Holland. The warblers and red-backed shrikes 
that have nested together in the fields of France, and that 
will winter together round the sources of the Nile or further 
south, will travel thither by different routes; the warblers 
may cross at Gibraltar, the shrike never will, but will go 
south by Italy to cross the Mediterranean at Sicily or Greece. 
The rose-coloured pastor and the black-headed bunting of 
the south of Europe will not get mixed with the millions 
that are making for Egypt from their nesting haunts, nor be 
diverted by the millions that cross their route making for 
the same region, but will continue to pass over Asia Minor 
through Afghanistan to winter in the North of India. The 
Arctic tern, that breeds right across North America to Alaska, 
never passes at the approach of winter down the Pacific 
coast, where it would find an easy subsistence, but crosses the 
Continent to pass down the Atlantic coast to winter along 
the coasts of Brazil, These and many other things are 
VOL. 11. 1M 
