Bordering on Extremes
Along our southern border, birds not seen elsewhere in the United States cope in a region that is growing hotter and drier
Dove song may be a beautiful sound, but last spring so many white-winged doves had staked out territory in San Antonio, Texas, that people began to complain. During mating season, the birds´ nearly ceaseless renderings of hhhooo-hoooo-hoo-hooo dominated entire neighborhoods.
The complaints were new to the area, and so was the noise. Until about 30 years ago, white-winged doves were almost never seen in the United States outside the remote thorn forests of far-south Texas or the Sonoran Desert of Arizona. For years after that, only a few wanderers were spotted further north. But in the middle 1980s, white-winged doves moved in great numbers into cities and towns as far north as San Antonio, Austin and Albuquerque.
"A lot of people think there´s been a population explosion among white-winged doves, but I´m suggesting that this is a redistribution based on loss of habitat," says Jeff Haskins, chief of the U.S. Fish and Wildlife Service´s migratory bird office for the Southwest.
White-winged doves are not alone. The fish- and insect-eating great kiskadee, the tiny Inca dove and even the shy, turkeylike plain chachalaca have all taken up residence in towns. And other birds--including the green jay, red-crowned parrot and green parakeet--also have moved northward in recent years. Scientists don´t know exactly why the birds are on the move, but they think that drier conditions, as well as an unprecedented loss of habitat, may be driving the birds out of their former territory along the U.S.-Mexico border and points south.
In the harsh climate of the border region--from the Texas Gulf Coast, through the mountains, deserts and plains of New Mexico and Arizona--birds have developed strategies over the millennia allowing them to withstand heat and to draw moisture from available sources. Now dropping water tables, drying rivers, dying streamside forests and disappearing brushland are testing the limits of those abilities.
Along the lower Rio Grande River, for example, birds once inhabited lush woods of elm, ash, sugar hackberry, sabal palm and Texas ebony. Now that habitat consists of dwindling patches of forest in narrow bands, and some birds--such as the Audubon´s and altamira orioles--are in trouble. At the Santa Ana National Wildlife Refuge, for example, annual numbers of altamira nests have declined by more than 50 percent since the 1970s. Audubon´s oriole has been added to the watch list of threatened species kept by Partners in Flight, an international, collaborative effort to aid birds and their habitats.
According to U.S. Fish and Wildlife Service (FWS) surveys, more than 95 percent of the original streamside habitat no longer exists. Much of the land was cleared for agriculture and development, and dams have eliminated flooding cycles that watered the forests. "The largest trees, the ones over 60 feet high, have died out because the water table has dropped," says FWS plant ecologist Chris Best. "The big old trees that used to have their roots in a permanent supply of moisture can´t reach it anymore."
Altamira orioles need big trees to hold and shelter their long, flexible nests. The location and the style of the nests help deter predators, but the strategy comes at a price. "The nests are so long they tend to sway in the breeze," says University of Texas-Pan American biologist Tim Brush. "If they aren´t in just the right place they can break loose and fall." To compensate, the birds nearly always build their nests on the northwest sides of big trees, shielding them from prevailing winds. But that tactic doesn´t work without tall trees.
When only thin strips of streamside forest are available, the orioles also are more likely to fall victim to bronzed cowbirds, which invade nests of other birds to replace eggs with their own. Since cowbirds favor open fields, they frequently parasitize other birds´ nests in trees near forest edges.
Other species that require dense forests for their survival are also in decline. The rose-throated becard, for example, a secretive, sparrow-sized bird related to flycatchers, no longer nests at all along the Rio Grande. Another songbird, the tropical parula, a small warbler that uses tall trees with Spanish moss to hide its nests from predators, also is vanishing from streamside habitat.
Although no one can say for sure whether the lower Rio Grande valley is hotter these days than in the past, there is no doubt that it has become extremely dry. Demand for water from agriculture, industry and rapidly growing urban areas has escalated to the point that little is left to sustain habitat for birds. "The water scarcity is the stress factor," says Robert Harriss of the Texas Center for Climate Studies.
As development accelerates and demand for water continues to grow, Brush and other biologists worry that more streamside, or riparian, habitat will be lost. As it is, only scattered patches remain throughout the arid West. "Riparian forests are the rivers of life that sustain great numbers of birds," says Arizona State University ecologist Robert Ohmart. "You have this tiny, tiny segment that is green and wet and productive. As you eliminate riparian forest, you eliminate a large number of birds that use it for breeding."
That´s why some species head for towns. "They´re some of the few places they can find tall trees and permanent water sources," says Lower Rio Grande National Wildlife Refuge biologist David Blankinship. "I´m up to about 92 species I´ve counted in my Weslaco backyard." The biggest surprise for biologists has been the presence in town parks of nesting chachalacas, which normally nest deep in the woods. "No one knows why some birds suddenly will begin to tolerate nesting around people," says Tim Brush. "Towns certainly aren´t a substitute for native habitat, but it´s encouraging that they are using those areas."
Still, the trend may not be all good news. "Towns have trees, but I doubt whether towns are a very good surrogate habitat for most birds," says ornithologist Scott Robinson of the University of Illinois. "What remains to be seen is whether they will have any breeding success once they move into town." In a recently published analysis of 30 years of data from the North American Breeding Bird Survey, Robinson concluded that songbird populations have stabilized in many parts of the country. Not so for the Southwest. "Everyone agrees that riparian areas of the Southwest are among the most endangered ecosystems in North America," he says.
Now, in addition to these concerns, scientists worry higher temperatures in the region that could result from global warming might stress birds beyond their tolerances. "Because a small bird´s evaporative water-loss rate skyrockets at higher temperatures," explains University of Arizona ecologist Blair Wolf, "just a one- or two-degree increase could double the animal´s water loss, and it just might not be able to acquire sufficient water to stay alive."
For the region´s historical extremes, however, birds are well adapted. Their typical high body temperatures mean they stay relatively cooler when it´s hot. During the heat of the day, they also seek out cooler, deeply shaded niches. In one novel strategy to stay cool, Gila woodpeckers and other birds nest in the saguaro cactus--the tall, ribbed plant with raised arms that has become the classic symbol of the American West. The high water content within often helps keep the nests cool. And after the woodpeckers move on to new saguaros, other birds, including the tiny elf owl, frequently adopt the cactus nests as their own. Verdins and cactus wrens beat the heat by building domed nests with covered roofs and tunnel entrances. The domes help shield the nests from the sun and provide insulation from the extreme cold of desert nights.
The keys to birds´ tolerance for high temperatures are adequate food and fluids. "If there´s no water to fly to, birds have to get all they need from the food they eat," says Wolf. For Gila woodpeckers, cactus wrens, verdins and even white-winged doves, that means a steady diet of insects or saguaro cactus fruit. But even with these adaptations, small birds may be especially vulnerable to spikes in temperature. There´s not much water in their bodies to begin with, and when the temperature rises to 118 degrees Fahrenheit or higher--and that is not uncommon in Arizona--a small bird can lose 5 to 7 percent of its body mass in one hour. Says Wolf, "At those temperatures you are approaching an animal´s dehydration-tolerance limits."
Mountain hummingbirds may be at similar risk, for somewhat different reasons. Because hummingbirds consume a steady diet of liquid nectar--hummingbirds regularly drink more than twice their body weight in fluids in a single day--they have evolved to excrete water as efficiently as possible. Hummingbirds simply aren´t able to hold onto adequate water in the event of drought or excessive heat.
Species such as the magnificent and blue-throated hummingbirds that inhabit a few mountaintops in Arizona and New Mexico may be at special risk. Increases in global temperature could dramatically alter the vegetation on which they feed. University of Arizona hummingbird specialist William Calder predicts that global warming likely will push plant zones higher in elevation. "Since mountains are conical, as you push up to higher elevations, there´s less area, so there´s less total habitat," he explains. If a plant that hummingbirds use already requires the highest elevations, it will be pushed off the top of the mountain.
As the birds look for other options, they may increasingly end up in southwestern towns. But for every new bird song that thrills or annoys people in urban areas, others are being silenced in the wild, where many species depend on conditions that can´t be substituted by parks and yards.
Santa Fe writer Vicki Monks has seen white-winged doves in San Antonio and Austin and is on the lookout for them in northern New Mexico.