• Sierra del Carmen

    Big Bend

    National Park Texas

A Lingering Doubt

Peregrine falcon

"The peregrine is without dispute the most nearly perfect flying machine in existence"

—Phillip Callahan, The Magnificent Birds of Prey

Peregrine Falcons in Big Bend National Park

On a cool summer morning I make my way up a steep-sided canyon in the Chisos Mountains. Overhead, white-throated swifts wheel and spin, their clicking calls alternately growing and receding as they dart across the sky. I pause to catch my breath, and happen to look up. In that instant, a dark shape plummets from high in the clear sky, gathering speed so rapidly it causes me to stop breathing. The dark shape rapidly takes form; a Peregrine falcon, wings folded, dives into the midst of the swifts. In a maneuver that happens so quickly I’m not sure I see it, the falcon strikes a solitary swift in an explosion of feathers, then circles gracefully to snatch the broken body before it can fall to the ground. Witness to a drama that has played out daily for centuries in the Big Bend, I stand in awe.

Visitors and researchers have come to the Big Bend for years to witness similar spectacles, especially since 1970, when the Peregrine falcon was placed on the federal Endangered Species list. By that time the falcons had all but disappeared from the eastern seaboard and were barely hanging on in isolated sites in the west. Persistent organochlorine toxins unleashed into the environment from widespread pesticide use, particularly DDT, caused eggshell thinning. The falcons failed to reproduce as their eggs crushed beneath them in the nest. In the Big Bend region Peregrine falcons have been recorded nesting since the beginning of the 1900’s. Through the 1970’s their reproductive success rivaled or surpassed the rate of populations in other western sites. The high and inaccessible cliffs of the Chisos Mountains and deep canyons of the Rio Grande provide safe haven for falcon eyries and bountiful hunting grounds. In recent years the National Park Service has annually closed areas to visitor use during the breeding season. This ensures solitude for the falcons in that critical time. In cooperation with the U.S. Fish and Wildlife Service, park staff has monitored the falcons yearly, compiling data on the reproductive success of this small and persistent population.

Elsewhere within the Peregrine’s vast range, similar monitoring revealed that the falcon has staged a remarkable comeback since that bleak day in 1970. The banning of the pesticide DDT in 1972, and successful captive breeding and release projects have returned the Peregrine to many former breeding sites and introduced it to new environments among skyscrapers. Today an estimated 1,650 breeding pairs range across Canada and the United States. On August 25, 1999, the Peregrine falcon was officially removed from the Endangered Species list. This was a success story that thrilled all who have followed the plight of this magnificent bird. Here in the Big Bend however, there remains a lingering doubt.



Beginning in the late 1980’s and accelerating between 1992 and 1996, productivity (the number of young/successful eyrie) of Big Bend Peregrines fell to alarmingly low levels. This period coincided with extended drought conditions in the area. A link to the low productivity was suspected. In 1997, 17 falcon young fledged, the highest number since monitoring began in 1971, and note was made that ’97 was a good rainfall year. Distressingly, drought conditions returned during the following two seasons and low fledgling success was observed. The impact of drought on wildlife populations is well documented, but concern remained that something else was playing a hidden, more sinister role in the falcon reproductive failure. The historical presence of mining in the region and continued agricultural practices upstream of the park provides sufficient reason to suspect that contaminants still affect Peregrines in the Big Bend. To address this concern, the U.S. Geological Survey, the National Park Service, and the Texas Parks and Wildlife Department conducted research to determine the levels of contaminants in potential prey species and their potential impact on Peregrine falcon reproduction.

During the summer of 1997, researchers began collecting samples from various prey species of peregrines. These included rough-winged swallows, black and Say's phoebes, cliff swallows, and bats. All of these creatures are insectivorous and employ different methods of capturing their insect prey, mainly along the river. They are all exposed to varying amounts of possible contamination. Tissue samples were analyzed and tests were conducted to detect concentrations of organochlorines, including DDE (a breakdown product of DDT), and several heavy metals, including selenium and mercury. The analysis revealed several disturbing trends.

Organochlorine compounds were not detected at levels that could affect survival or reproduction of Peregrine falcons, except DDE. Of the five species sampled, only the northern rough-winged swallow contained levels of DDE that could cause eggshell thinning and reduced reproductivity in falcons. Significantly, the level of DDE in the swallows was five times above the threshold, the level above which serious physiological effects begin. Higher DDE levels in northern rough-winged swallows may be accounted for by differences in diet and foraging. Northern rough-winged swallows are more likely to feed over water and skim insects from the surface than the other species sampled.

The heavy metals selenium and mercury were also detected at levels of concern. Selenium levels at five micrograms per gram of sample weight could result in embryo deformities. Selenium concentrations were at threshold in rough-winged swallows and approached threshold in the phoebes. Mercury levels of 0.5 micrograms reduced egg laying and hatching in mallards, and at 1.2 micrograms reduced clutch size, increased nest desertion and reduced nesting territory in common loons. In this study, all five sample species had mercury levels above threshold for protection of fish-eating birds (threshold levels of selenium and mercury have not been established for Peregrine falcons). Research results reveal that the persistent and toxic compounds, selenium, mercury and DDE, are accumulating in the food chain along the Rio Grande. They may be implicated in reproductive failures and reduced reproductive success of the Peregrine falcon in the Big Bend. Having returned so far from the brink of extinction, Peregrines still face challenges to their survival. This "nearly perfect natural flying machine" still merits our protection. Through research and application of findings the solutions for their continued presence may be found.

Did You Know?

Woody Guthrie

American folk singer Woody Guthrie spent a short time in the Big Bend region in 1921 and briefly searched for the legendary "lost mine" in the Chisos Mountains. More...