What You May Learn on Bird Trek Arizona
(Chalk Talk Topics)
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.
Chalk Talk 1, at Tucson Mountain Park:
Why a desert is here
Chalk Talk 2, on Mt. Lemmon:
Why mountains are here
Chalk Talk 3, in Cave Creek Canyon: Why there are so many kinds of birds
Chalk Talk 4, in Cave Creek Canyon: Why there are so many kinds of birds in SE AZ
Chalk Talk 5, at Rustler Park: How range expansion happens
Chalk Talk 6, at Reef Townsite: What limits range expansion
Chalk Talk 7, at Patagonia Lake: What happens when ranges collide
Chalk Talk 8, at Patagonia Lake: Why the names of species keep changing
Why a desert is here
The curvature of the earth causes the Equator to receive more solar energy than other latitudes.
This causes moist equatorial air to rise, dropping its moisture as rain, creating the Tropical Rainforest BIOME.
The dry air flows toward the poles at the base of the stratosphere, then subsides at 30° N and S Latitudes, where it dessicates the landscape, creating the Desert Biome.
Tucson’s latitude is 32.22°N. The Sonoran Desert stretches north to Phoenix and south to Guaymas.
Different biomes develop under different combinations of temperature and rainfall. A given biome is replicated around the planet wherever conditions are right.
Why mountains are here
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.
Why there are so many kinds of birds
Competition for food has led species to specialize, evolving highly efficient structures and behaviors.
e.g., webbed feet and swimming, hovering and nectar-feeding, chisel-shaped bills and excavation
Different biomes provide different resources, and many species specialize on a niche in a single biome.
e.g., meadowlarks in grassland, chickadees in forest
The different continents of the world were isolated for a long time, allowing independent (and "convergent") evolution of the same niches and biomes with different players, creating REALMS.
e.g., penguins and auks, old world and new world vultures, hummingbirds and sunbirds
Why there are so many kinds of birds in SE AZ
LOCATION: 30 N. Lat. The closer to the Equator the more species a place has.
LOCATION: Topographic diversity increases the numberof biomes from 1 to 4
Desert, Grassland, Woodland, Forest
Plus rivers and canyons create microclimate habitats
LOCATION: BioGeographic crossroads
Rocky Mountain forests to the north, Sierra Madre forests to the south
e.g., chickadees. Many differences at the subspecies level
Sonoran Desert to the west, Chihuahuan Desert to the east
e.g., ravens. Some subspecies differences, several Chihuahan species recovering
LOCATION: The Gadsden Effect. Many Mexican species are expanding northward, and we notice this because they
become part of the avifauna of the U.S.
This may be a continuation of post-glacial range "reclamation."
Some attribute it to global warming, but it's worth noting that ranges are expanding southward, too.
How breeding ranges expand
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.
Migration evolves (easily) when the breeding range is not useful during the winter.
What limits range expansion
Usable habitat is non-existent outside the occupied range.
Individuals may die "looking," or
Unusable habitat is not convertible,
i.e., population is unable to adapt to unfamiliar conditions, or
Physical barriers to dispersal prevent reaching a usable habitat patch, or
Ecological and/or social “resistance” prevent colonization of a new habitat patch.
What happens when ranges collide
One or both populations continue to expand and the two become sympatric, or
The range expansion stops where the two populations meet, or
One population expels the other behaviorally or swamps it genetically, or
The two populations interbreeding widely, resulting in
One species, or Three species, or A hybrid zone in which the hybrids outcompete both parental species.
Why the names of species keep changing
Thanks to the revolution in molecular biology, scientific knowledge of relationships advances rapidly, requiring revision of the tree of life.
The notion of "species" does not align well with the branching topology of trees.
The competing species concepts are all strictly bifurcational,
while intensive research indicates that Audubon's Warbler, for example, is of hybrid origin.
But, we still need names, so a protocol must be followed.