The peregrine falcons are returning to Big Bend National Park for their breeding season, and the Park, as they have done in previous years, has closed a small section of the South Rim trail of the Chisos Mountains to hikers, to give the birds a quiet and safe place to raise their young.
Every February, Raymond Skiles - the Park's wildlife biologist - hikes up to the South Rim with trail closure signs, to keep hikers from getting too close to the peregrine falcon’s nesting area.
From the edge of the South Rim, Skiles points out the bird's nesting sites, "From here, for the next quarter mile at least, there’s various points where historically the birds have laid their eggs."
Nesting in these cliffs, high above the desert floor, the peregrine can keep an eye on everything below it, including the birds it preys on. Dropping from the cliff-top, the peregrine reaches speeds over 200 mph, slamming into its unsuspecting prey with fisted talons.
"That kind of impact, with a stiff arm - well legs in this case - fist, just having impact on the prey bird, stunning it, knocking it out, causing it to just start tumbling," says Skiles.
The peregrine then catches the falling prey in mid-air, and carries it back to the cliffs.
However, the peregrines can’t see up behind them to the top of the cliffs. This leaves them vulnerable from above. They can be easily frightened by unexpected noises, such as those made by hikers, causing them to fly away and leave their eggs alone on the cliff.
"If they have eggs and maybe its a chilly day or something, those eggs can get too cold to survive, and thats really the purpose of this restriction of this part of the trail," explains Skiles.
The park has been closing this section of the trail for the nesting season - February through May - every year since the first nesting site, or eyrie, was discovered back in 1985. Skiles was there that day.
"My coworkers and I had gone down on this point, to have lunch, and had been told that peregrines had been seen in the area, but nobody knew where they were nesting. We saw one fly in, went right to a ledge, and picked up binoculars and sure enough there was the adult with 3 downy checks, " he recalls.
Skiles and his coworkers began keeping a close eye on the bird. The peregrine had been on the endangered species list since 1970. Widespread use of pesticides like DDT in the 1960s had contaminated the environment and entered the food chain. Because the peregrine is at the top of the food chain, it suffered the highest concentration of the pesticides.
"If there’s water that has contaminates, and the insects in that water would accumulate those contaminates, and then maybe birds that eat those insects would even further accumulate, and then the peregrines that ate those birds that ate those insects that were in that water would have the highest doses in their tissue," explains Skiles.
These contaminates were proven to have a terrible effect on the bird’s eggs - they caused thin shells.
"When the parents were incubating, that is lowering their bodies on the eggs, gently as it were, they would crack those eggs because they were unnaturally thinned shells. And thus the peregrine population plummeted," says Skiles.
The banning of these chemicals, and the protection of existing peregrines and their eyrie sites, has allowed the bird to recover. It was taken off the endangered species list in 1999. But the park continues to monitor the nesting sites.
"Keep a finger on the pulse you might say," says Skiles, "maintain the vigilance incase something else starts affecting and the harming the birds. We would be able to respond more quickly to it if we were actually keeping tabs on them."
The peregrines return year after year to reunite with the same partner. As long as they keep coming back, the park will make sure they have a peaceful site to raise their young. But, Skiles says- visitors can still enjoy stunning views from much of the South Rim trail. The full trail will reopen June first.
- Ian Lewis