After decades of being trapped and shot out of the skies as vermin and having their food poisoned by organochlorines like DDT, birds of prey in North America have made a remarkable comeback.

Thanks to protective laws and the banning of harmful chemical pesticides, bald eagles and peregrine falcons have had their "endangered species" status removed both in the U.S. and in Canada. However, there could be dark clouds on the horizon for all raptorial birds, especially peregrines.

Last week I received an email from Bud Anderson in Seattle, Wash., who witnessed the strange deaths of all three young, at around 2 weeks of age, in a peregrine nest on a building in that city. Daniel Varland, another colleague of mine in Washington, reported that two of four peregrine nestlings suddenly died for no obvious reason in a cliff nest. In a third case, Bob Walters of the Utah Division of Wildlife Resources, discovered three peregrine nestlings dying mysteriously in a nest on a building in Salt Lake City, Utah.

Unseasonable weather, such as hot temperatures or severe rainfall of the kind that led to fatal hypothermia and possible drowning of the three young on a Winnipeg skyscraper in late May, have been ruled out. Also, it is unlikely, but not impossible, that the nestlings succumbed to a pigeon-transmitted parasitic disease called frounce or trichomoniasis, if only because the adults should have been affected similarly. The same logic might be applied to the suggestion that illegal pigeon-poisoning programs are to blame. In any case, at least some of the carcasses have been sent off for necropsies to Lindsay Oaks, a veterinary pathologist at Washington State University in Pullman, Wash.

I contacted Greg Septon, who manages a large population of urban-nesting peregrines in the midwest U.S., but he has not observed any strange nestling deaths. On the other hand, he is puzzled about the sudden disappearance of partial clutches of eggs from four of his nests, as well as a clutch of eggs that failed to hatch.

Here on Montreal Island, we have no less than 10 peregrine territories, including the major bridges. While I cannot speak for the bridge and quarry pairs for this year, the Place Victoria pair, under the watchful eye of lawyer Jean Masson, failed this year. They laid at least two eggs, one of them white and thin-shelled, which they abandoned. Masson suspects that aggressive intervention by a third peregrine could have caused the failure.

A brand-new pair for 2008, which have adopted the tall tower housing the archives on the campus of the Université de Montréal, also met with disaster. During the installation of a second nesting tray on the tower, Eve Belisle, a guardian of the falcons at l'École polytechnique , discovered some eggshell fragments and encountered very weak defensive behaviour by the female, indicating the absence of chicks.

But there could be another reason accounting for the nesting failures. Three words are on the lips of most raptor biologists these days - brominated flame retardants. A recent article from the Los Angeles Times stated that peregrine falcons sampled in major cities in California have been discovered to contain record levels of polybrominated diphenyl ethers (PBDEs) in their bodies. Alarmingly, this parallels high levels of PBDEs found in the breast milk of Californian women. PBDEs are the main component of flame retardant chemicals that are applied to most of our household products, e.g. electronics, textiles, and even automobile parts to reduce combustion risks and enhance safety.

When PBDEs leach from these products during disposal and even during normal use to reside in household dust, they soon enter the food chain and accumulate in top predators in the food chain, including birds of prey and humans. These chemicals are known as endocrine-disruptors. At the Avian Science and Conservation Centre on McGill University's Macdonald campus, I am one of the leaders of a research team that has been studying the influence of PBDEs on American kestrels, a close cousin to the peregrine, for four years. To date, we have discovered impacts on thyroid and reproductive function.

Even more scary is the recent finding that after as much as two years later, embryonic exposure to PBDEs for just 28 days has had a negative impact on the courtship behaviour, reproductive system, and fertility of adult male kestrels paired with clean, unexposed females.

While that's bad enough in itself, another question is ... what are PBDEs doing to humans?

David Bird is a professor of wildlife biology and director of the Avian Science and Conservation Centre on the Macdonald campus of McGill University.