I want to give you a chance to understand why Southeastern Arizona is so special
for birders, and beyond that, why the earth's birds are where they are. And so, with our afternoon cocktails we will have very brief introductions
to the following subjects. Should there be questions, well, we have the next 24 hours to answer them. Follow the blue links to the main points of each topic.
The "mountain islands" of the Basin and Range Province were created when the lowlands that comprise the "desert sea" were stretched by tectonic forces, beginning 30 million years ago.
The cracks in the surface evolved into faults, and thousands of feet of crust slid upward along the faults, creating fault block mountains such as the Catalinas and Huachucas, while also spawning volcanoes
This episode was caused by the subduction of the Farallons Plate under the North American Plate, which had previously created the Rocky Mountains.
Temperature decreases and precipitation increases as one goes up a mountain slope, permitting the existence of several Biomes (life zones) on one mountain.
"Rain shadows" occur at lower elevations on the leeward sides of mountains, for the same reason that deserts occur at 30 degrees latitude (warmer air [downslope] can hold more moisture than colder air [upslope]).
Successful reproduction causes local populations to grow.
Natal dispersal of individuals produced near the edge of a population expands the footprint of that population.
Expansion of populations near the edge of the range of the species causes the range to expand.
Habitat gaps (e.g., water, poor habitat) are bridged when the climate changes and the gap is converted temporarily to breeding habitat (e.g., Bering Land Bridge).
Migratory movements of birds seldom establish new populations. Nonetheless, habitat gaps are occasionally bridged without a habitat conversion. The relative contributions of dispersal and migration to this success is an open question.
Migration evolves (easily) when the breeding range is not useful during the winter.