Present distributions are the result of previous history plus taxonomic history. There are few cosmopolitan species because 1) ecological limits are restricted and 2) barriers to dispersal exist. You might think birds would ignore barriers, but they show regional fidelity despite their flying potential. They are reluctant to cross unsuitable habitat. The best dispersers are in temperate zone.
The HOUSE SPARROW is in Death Valley, having dispersed across the whole country, but the BROWN TOWHEE, also urban, is poor at dispersal--the Farallon Islands, 28 miles from mainland, lack Brown Towhees.
The theory of continental drift revolutionized Zoogeography. There have been many changes through time: i.e., New World Vultures were in the Old World. Mountains have risen; Earth is in a constant state of flux, and we see only a second.... Wegner postulated a single Permian landmass: Pangea. Continental plates move out across the earth's surface (landbridges were used as an explanation for distribution before Continental Drift).