CHAPTER IV 



THEIR MEANS OF DISPERSAL AND MIGRATIONS 



The dispersal of birds — Fortuitous dispersal — Ancient land- 

 surfaces — Polar dispersal — Equatorial dispersal — The 

 law of dispersal — Obstacles to dispersal -- Discon- 

 tinuous areas of dispersal — Influence of climate upon 

 species — Species increasing their range during present 

 time — The migration of birds — Misconceptions con- 

 cerning migration — Transmutation and hibernation — 

 Migration and instinct — Migration not universal in 

 certain species — The Cause of migration — Migration 

 due to range expansion or colonisation — Cause of 

 migration in autumn — Length of journey variable — 

 The philosophy of migration — The wings of migrants 

 — Moulting of migrants — Course of autumn migration 

 — How birds find their way on migration — Migration 

 across seas — General aspects of migration — Duration 

 of migration — The speed of migration flight — The 

 height at which migrating birds fly — The perils of 

 migration — Lost birds at lighthouses — Various kinds 

 of migration — Vertical migration — Local movements 

 of birds — Nomadic migration — Irruptic movements. 



The two preceding chapters having dealt with 

 the principal groups into which birds are 

 divided by systematic ornithologists, and the 

 various parts of the globe over which they 

 are distributed, it becomes necessary now to 



