Birds and Birding's Guide to:

Watching GEOGRAPHICAL DISTRIBUTION OF BIRDS

Glacial Effect of Bird Distribution

Another and very important geological event was the glacial epoch. This vast ice-mass, sweeping down for hundreds of miles from the pole, profoundly modified the life, not only in the area actually covered by the ice, but far beyond its actual border. Many forms must have been crushed out of existence, while others, enjoying perhaps better means of migration, were pushed before it towards the tropics, which resulted in intensifying the struggle for existence in an area probably already well stocked.

The forms of life from both sources that could not readily adapt themselves to the changed conditions were pushed to the wall and left little or no trace of their existence. By the recession of the ice, territory was gradually reclaimed, which was occupied by the surviving forms, with the exception of those typically northern forms remaining permanently stranded in southern mountain areas.

To paleontology we are also indebted for some contributory data regarding distribution, for while the fossil remains of birds are not very numerous, they are often sufficiently so to show that many groups once enjoyed a much wider distribution than now. Paleontology thus makes plainer the possible lines of travel by which the descendants of the ancient forms have reached their present locations.

But this class of facts is far less important than those last considered, for the paleontological record is less complete for birds than for almost any other| group. It is mainly of value in fixing the antiquity and affinities of certain groups, and even here it is often distressingly meager.

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