DISTRIBUTION OF BIRDS 27 
acquaintance with birds without a formal attempt to add to our knowl- 
edge of them. Bird study may not necessarily be anything so serious 
as a study; it may be merely a recreation, a pastime, even a ‘fad,’ 
if you like; but so long as our interest in birds is sufficient to take us 
to their haunts, or so long as the voice of a bird expresses for us that 
joy in nature which is the rightful heritage of every human being, 
just so long will it repay us to add to our sources of pleasure that knowl- 
edge of birds which will permit us to “come at these enchantments.” 
Tue DistTRIBuTION or Brrps 
Factors Influencing Distribution 
Faunal Areas 
Zones of the Boreal Region 
Zones of the Austral Region 
Floridian Fauna 
Factors Influencing Distribution.— Possessed of a space-defying 
means of locomotion, birds are more widely distributed than any other 
vertebrates; but in spite of their unexcelled mobility their sensitive 
organizations respond quickly to those influences which determine the 
distribution of life. Consequently we find that while some species 
have an almost world-wide range, others are confined to surprisingly 
restricted areas. The factors determining the boundaries of the region 
inhabited by any given bird may be classed primarily undef*the heads 
of Past and Present. (Past factors include those great earth-forming 
forces which, through a series of profoundly important changes, have 
brought about the now long-standing inter-relation of land areas—in 
other words, the world as we know it. The land bridges which connected 
Great Britain with the Continent, or Alaska with Siberia, and the 
strait which separated the American continents at Panama are factors 
of this kind. In their time they obviously exercised a powerful influence 
on the distribution of life. Were we equally sure of all the land con- 
nections and water separations which have existed since life appeared 
on the earth, we might hope to solve many at present inexplicable 
problems in distribution. 
Of far-reaching importance also has been the evolution of climates 
which this globe-has witnessed, and which, through the last Glacial 
Period, has introduced the climate under which we now live. Reference 
to page 57 will explain how the distribution of White Pelicans, 
for example, is believed to have been affected by such past climatic 
changes. 
Climate, of course, has never ceased to exert its influence on the 
distribution of life and we find it the most active present-day factor. 
It is expressed mainly through temperature, and, to a lesser extent, 
through rainfall. Thus the boundaries of the three transcontinental 
zones forming the Austral Region (see cover map) conform more or 
