Once nearly wiped from the wild in North America by widespread pesticide use, Peregrine falcons have rebounded across the continent. Now, scientists are studying whether the predators are running into trouble from BP's oil spill.

The research may also help determine the health of species lower down the food chain in the Gulf of Mexico.

The falcons, along with bald eagles and brown pelicans, nearly vanished from the wild before the pesticide DDT was banned in 1972. Since predatory birds are at the top of the food chain, smaller amounts of DDT that tainted their prey built up to higher dosages in their bodies and caused egg shells to thin, threatening reproduction.

All three species have since been removed from the federal endangered list, but scientists are concerned that some migrating Peregrine falcons passing through the Gulf from their nesting grounds in Alaska and Greenland may be affected by remnants of the oil spill.

"They look for a prey species that's handicapped in some way, and they'll go after that bird, so if they would see a shore bird that has oil on it, that's not flying quite right, as a predator, they'll key in on that bird, and kill it first," said Bill Heinrich, a biologist with The Peregrine Fund, a nonprofit group based in Boise, Idaho, that helped restore the birds in the wild through its captive breeding program.

Heinrich worries toxins from the oil spill may wind up contaminating falcon prey and could accumulate in their systems and cause reproductive problems when they return home. He also says if any toxins are found while collecting blood samples from the birds currently migrating across the Gulf, it could be an indicator that other species aren't faring well.

"Being at the top of the food chain, they're an environmental barometer ... You know if something happens to the Peregrine, you know that things further down the food chain, things aren't right," Heinrich said.

The Peregrine Fund and other scientists are currently capturing the birds on South Padre Island in Texas, taking blood samples and releasing them back into the wild to make their journey home. It could be several months before test results indicate harmful effects from the oil, if any.

Roger Helm, chief of environmental quality for the U.S. Fish and Wildlife Service, says it's probably unlikely that the falcons will suffer any long-term effects given BP's well that blew out on April 20 spewing more than 200 million gallons of oil has now been capped, and consistent exposure would be needed to severely harm the birds.

However, he said, the research is certainly welcomed and could help answer some unresolved questions.

"Given the fact that it's been hard to figure out where all this oil is, if it's showing up in really high levels in Peregrines, wow, that would be very significant," Helm said. "Even in just a few birds. That would be very suggestive that there's some real significant source of oil out there, and that would be useful information."