Birds and the Border

Diane and I have visited the border between the US and Mexico a number of times in the last few years.  One reason we go to these areas is because a lot of interesting birds reach the northern end of their range in the borderlands and are hard to find in other parts of North America.

Plain Chachalaca (Ortalis vetula) is a tropical bird that reaches
the northern end of its range in South Texas.  Photographed
at Estero Llano Grande State Park.

South of San Diego, California is the busiest port of entry in the US.  This border crossing connects suburban San Diego and Tijuana, Mexico.  Millions of vehicles and many more millions of people cross there every year.  The border wall is very prominent in this area.  Metal slats, in some areas topped with razor wire, restrict the flow of people.  The barrier even runs into the Pacific Ocean.  Customs and Border Protection’s (CBP) white and green-striped trucks are common in the area and agents ride jet skis a half mile into the ocean to look for swimmers trying to go around the wall.  While birding along the Tijuana Slough just north of the border in California, we saw our first White-tailed Kite (Elanus leucurus) and Long-billed Curlew (Numenius americanus). 

Border wall between the US and Mexico extending
into the Pacific Ocean. Viewed from Tijuana, Mexico.

Some of the nation’s premier birding spots are in the mountains of southeastern Arizona.  These sky islands rise from the Sonoran and Chihuahuan deserts and host many bird species that are at the northern edge of their range.  Painted Restart (Myioborus pictus), Mexican Jay (Aphelocoma wollweberi) and Elegant Trogon (Trogon elegans) are all species that just reach the US in these mountains.  

Elegant Trogon (Trogon elegans) in Cave Creek Canyon, Arizona.

Painted Redstart (Myioborus pictus) in Maderia Canyon, Arizona.

The Huachuca Mountains lie just a few miles from Mexico and border security is ever-present.  The canyons of these mountains are renowned for their bird life.  One of these canyons also hosts Fort Huachuca, established in the 1870s as a cavalry outpost in the Indian Wars.  In addition to the ubiquitous CPB vehicles, this area has another interesting border security device. On most days, an aerostat, a blimp-shaped balloon, flies over the fort on a tether.  We met a couple of soldiers from the base while hiking and they told us the aerostat has radar and other sensors to watch the border.  

The Fort Huachuca aerostat, visible as a white dot
to the left of the pine. This blimp-like ship has sensors
to monitor the US-Mexico border. 
Carr Canyon, Arizona.

Closer view of the aerostat. 
Carr Canyon, Arizona. 

Mexican Jay (Apheolocoma wollweberi) in Ash Canyon, Arizona.

The Rio Grande valley in Texas is another area where rare birds abound at the border.  This fall we went with a group that birded from near the mouth of the river to the fabled cowboy town of Laredo.  The lower end of the Rio Grande valley is subtropical.  Native palms are abundant in the few remaining natural areas and the region is a major citrus producer.  The birds here also have a tropical flavor about them.  Of the 181 species of birds our group saw, I had previously seen 15 of them in the American tropics.  Some of these tropical birds were; Tropical Kingbird (Tyrannus melancholicus), Great Kiskadee (Pitangus sulfuratus), Neotropic Cormorant (Phalacrocorax brasilianus), Plain Chachalaca (Ortalis vetula) and Ringed Kingfisher (Megaceryle torquata).  The tropical affinity of the region extends to its insect life.  South Texas has its own leafcutter ants (Atta texana).  These ants march from their nests, cut sections of leaves from live plants and carry the leaf parts back to the nest.  At the nest, the ants chew up the leaves, inoculate the macerated plant tissue with a fungus and then eat the fungus.  These ants are farmers.  

Texas Leafcutter Ants (Atta texana) at Frontera Audubon, Texas.

Another Lower Rio Grande Valley location we visited was Sabal Palm Sanctuary in Brownsville, Texas.  The Sanctuary was actually located on the other side of the border wall.  Sabal Palm is in Texas so we did not need our passports and did not enter Mexico.  We drove over a dike and through a gap in the rusty slat wall to the Sanctuary.  At Sabal Palm, we got our first looks at the Altamira Oriole (Icterus gultaris) and Olive Sparrow (Arremonops rufivirgatus).  If the border wall at Sabal Palm became as impervious as the wall near San Diego, the Sabal Palm Sanctuary, homes, farms and cemeteries would be lost in a no man’s land between the border and the wall.  

Border wall, Sabal Palm Sanctuary, Texas.

For much of the length of the Rio Grande in Texas there is no wall.  A few hundred feet of water is all that separates Mexico from the United States.  Much of the border is rural with farms or scrub forest on either side of the river.  These areas still have a CBP presence.  Cleared riverbanks, dikes with roads on their tops, towers with floodlights and cameras are scattered along the US side of river.  

Altamira Oriole (Icterus gularis) at Salineno Preserve, Texas.

Altamira Orioles on Customs and Border Patrol
floodlights, San Ygnacio Sanctuary, Texas.

The Juarez-Lincoln Bridge links Laredo, Texas, USA to Nuevo Laredo, Tamaulipas, Mexico. As traffic waited on the bridge to enter the US, our birding group saw two Ringed Kingfishers, the largest kingfisher in the country in Laredo.  One of these birds flew across the river and it was no longer an American kingfisher but a Mexican one.  Birders are particular about where they see a bird.  Some go so far as keep lists for counties, states, countries and the world.  We could not count birds on the Mexican side of the Rio Grande for our Texas bird list.  

Ringed Kingfisher (Megaceryle torquata) at Zacate Creek, Laredo, Texas.

We saw dozens of birds cross the river between the two countries.  Green Jays (Cyanocorax yncas), White-tipped Doves (Leptotila verrauxi), Great Kiskadees and Gray Hawks (Buteo plagiatus).  All made the crossing at Laredo in seconds while drivers on the bridge waited an hour to cross.  

Great Kiskadee (Pitangus sulphuratus) at Quinta Mazatlan, McAllen, Texas.

Green Jay (Cyanocorax yncas)
 at Laguna Atascosa National Wildlife Refuge, Texas.

The southern border, like all international frontiers, is a political boundary imposed on an ecological landscape.  What a sharp contrast between the ease with which birds cross the border and the complications to people crossing.    

Iron and Water

Sometimes in a swamp or marsh, you can see a metallic sheen on the surface of the water.  It may look like an oil spill but it is usually a molecular scale layer of iron floating on the water’s surface.  You can tell oil on water from iron on water by touching the surface.  If the film breaks up into plates, it is metallic.  

Iridescent layer of iron produced by bacteria floating in a marsh in Clayton County, Georgia.

If the layer swirls and reforms, it is oil.  The basis for the sheen with both oil and iron is thin film iridescence.  This type of iridescence is found not only with oil on water and iron on water but also in soap bubbles.  The thickness of the film determines which colors are reflected, so oil spilled on a wet road or soap bubbles may show all the colors of the rainbow. 

Plates of iron bacterial film broken up by water movement. 

Oil on pavement showing thin film iridescence.

This iridescent layer is the signature of a bacterial ecosystem that runs on different ionic forms of iron.  The sheen appears in still water when there has been no rain for several days.  Rain or water current will break up the layer into plates and they will wash away.  In these swampy places, decomposition of plant material by bacteria and fungi deplete the oxygen in the water.  This anaerobic environment is where a several bacteria use iron in their metabolism.  Animals, plants and most fungi are aerobes and carry out their energy metabolism by removing electrons from organic molecules like sugars.  These electrons go through a bewilderingly complex set of chemical reactions to make cellular energy.  In the last step of this process, the electrons end up on oxygen.  Bacteria involved in making the iron sheen have a similar type of energy metabolism but rather than dumping their electrons on oxygen, they put their electrons on the oxidized form of iron.  This oxidized iron is abundant in the red clay soils of the southeastern Piedmont.  The iron that has gained those electrons is said to be reduced, an odd term since the atoms have actually gained something (electrons). 

Bacterial iron film.

Reduced iron is an energy source used by another group of bacteria, the iron oxidizers.  These bacteria remove the electrons from the reduced iron and regenerate the oxidized form while producing their energy.  Iron oxidizing bacteria live at the top few millimeters of the anaerobic swamp water where a small amount of oxygen is dissolved.  The bacteria deposit the iron produced by their metabolism as iron oxide, also known as rust, at the surface of the water and make the shining layer.  These two groups of bacteria depend on the metabolism of each other to fuel their survival.

If you run across iridescence in a marsh, it probably does not signal a petroleum release but rather a sign of an iron-based microbial ecosystem.