At first, scientists believed that this was an accidental phenomenon. However, they later noticed that the American hawk, the smallest bird of its kind in the country, was beginning to disappear at an alarming rate. At the same time, the population birds endangered species such as vultures and bald eagles have begun to increase.

“It’s a mystery,” said Chris McClure, who leads global conservation research for the Peregrine Fund, a wildlife conservation group.

Initially, residential development combined with agricultural exploitation were considered as possible causes. So they placed artificial nests to attract females of the species, and the trick “seemed to work.” But over the years, many nests have become empty, which again raises the question of what harms falcons. However, their population was large enough not to be considered an endangered species.

There are many speculations about this abbreviation. “Some are standing,” Dr. McClure said. Previous predator disappearances have been solved as murder mysteries. DDT, an insecticide, has been the most responsible for many years. After being banned in the US in the 1970s, bald eagles and falcons have made a comeback.

Decades later, another raptor extinction event occurred when three South Asian vulture species began to lose over 95% of their population within a decade. The birds seemed doomed until scientists discovered what was killing them: a cattle painkiller that became widely used. The vultures ate it like they ate dead cows.

Another example is the American bird of prey, Swainson’s hawk. Little was known about its winter migration, and no one could figure out why the species was declining until the scientist installed satellite transmitters on two birds he later discovered in Argentina. Following them through sunflower fields, he found hundreds of dead hawks. According to the farmer, the birds died after spraying the ground. This time, the culprit was an insecticide called monocrotophos.

But so far, attempts to find out who is to blame for the decline of the falcons have been unsuccessful. Although it appears that the decline may have stabilized, scientists are concerned. Since 1970, North America has lost a total of almost 30% of its birds, according to research.

“So Many Factors”

About 30 years ago, John Smallwood, professor of biology at Montclair State University, installed 100 birdhouses in one of the best hawk habitats in New Jersey, attaching them to poles and trees near fields and pastures. In the first year of 1995, the hawks built three nests. The next year there were seven, and then 26. By 2002, there were 61. But then the number of nests began to decline. Last year there were only 21.

Dr. Smallwood and two graduate students record each bird’s data: ring number, weight, moulting status. A plucked feather yields DNA. The history of bird life has provided interesting observations, if not answers to some questions. Elderly females who choose mates do not seem to prefer inexperienced mates. “They know better,” Dr. Smallwood said with a laugh.

Overall, the study shows that the environment appears to be satisfactory and the birds on display are doing well. “The problem is they just don’t show up,” he added.

Little is known about where migratory bird populations spend the winter or what happens there, although research in Texas and elsewhere is trying to answer some of these questions. Scientists can’t mount satellite probes on falcons, as they did with Swainson’s larger falcons, because they are too small.

In an article in The Journal of Raptor Research, Dr. Smallwood and David Byrd, professor emeritus of wildlife biology at McGill University in Montreal, list possible factors in the reduction of the number of falcons, which, in their opinion, require additional research.

Could the growing population of Cooper’s hawks limit the habitat of the American peregrine falcon? What happens in the winter habitat of falcons? Could the extinction of hawks be related to the extinction of insects? Is it because of the mice and rats that ate the poison? What are the effects of neonicotinoids, a particularly strong insecticide? What about the consequences of climate change?