The Mystery of the Vanishing Kestrels: What’s Happening to This Flashy Falcon?
Catrin Einhorn covers biodiversity for The Times. As part of her reporting, she spent a day with researchers climbing ladders to check kestrel nests. At first, people thought it might be a housing shortage. Scientists had noticed worrisome declines in the American kestrel, a small, flashy falcon found coast to coast. The downturn was especially puzzling because birds of prey in North America are largely considered a conservation bright spot. Take bald eagles, whose population in the contiguous United States . Or red-tailed hawks, which sometimes nest in city buildings, dining on mice and rats. Turkey vultures are on the increase, too. Why are all these other raptors doing great when the American kestrel is on the decline? said Chris McClure, who directs global conservation science at the Peregrine Fund, a conservation group. Maybe the problem was a lack of nesting spots, some researchers thought, as intensive agriculture and ever more housing developments meant fewer dead trees in the open landscapes that kestrels need. Scientists and set out nest boxes, and kestrels moved in. But over the years, many of the boxes emptied, complicating the question of what is hurting kestrels, North Americas smallest falcon. They are still common enough that theyre not considered threatened on a national level. But scientists estimate that the continent has lost half its kestrels since 1970. Those in the east have fared worse. Hypotheses about the decline abound. Everyones got their own favorites, Dr. McClure said. Previous raptor disappearances have been cracked like murder mysteries. DDT, an insecticide that causes eggshells to thin, is the most famous culprit. After it was banned in the United States in the 1970s, bald eagles and peregrine falcons rebounded. Another case came a couple of decades later, when three species of South Asian vultures , a confounding crash of more than 95 percent of the population within a decade. The animals seemed doomed to extinction until scientists : a painkiller for cattle that had recently come into wide use. Vultures ingested it when scavenging dead cows. A third example involved an American bird of prey, the Swainsons hawk. Little was known about the birds winter migration, and no one could figure out why they were declining until a scientist attached satellite transmitters to two individuals, tracking them to Argentina. Following them to sunflower and alfalfa fields there, he found hundreds of dead hawks. The birds had died, a farmer said, after the land was sprayed. The culprit this time was an insecticide called monocrotophos. Advocates to remove it, and the hawks stabilized. But so far, efforts to find a smoking gun for kestrel declines have been inconclusive. While some research indicates that declines may be leveling off, scientists are alarmed. The broad backdrop for birds is not good. Overall, North America has lost , research shows. Weve been really good at killing the things that are common, said Julie Heath, a biology professor and kestrel researcher at Boise State University in Idaho, pointing to passenger pigeons as an avian example. It just seemed like, Hey, we can shoot these things for forever and theyre never going to be gone. And then, all of a sudden, they were. About 30 years ago, John Smallwood, a professor of biology at Montclair State University, set up 100 nest boxes in what should be some of New Jerseys prime kestrel habitat, attaching them to utility poles and trees near fields and meadows. The first year, 1995, kestrels made three nests. The next year, there were seven, then 26. By 2002, they reached a high of 61. But then the number of nests started dropping. Last year, there were only 21. This year again, the vast majority of his boxes sit empty of kestrels. On a recent afternoon, some contained only wood chips. One had been claimed by bluebirds. The kestrel nests that do exist are carefully tracked. Heres how to catch a kestrel in a nest box: Approach slowly, ideally masking your sound with a passing car or plane. One person covers the nest hole with a net while another climbs a ladder and unlatches the box from the side. (This is all illegal without federal and state permits.) Hello, Dr. Smallwood whispered to a leg-banded female who remained calm in his hands after he lifted her off her eggs. Were old friends, we know each other from way back. Dr. Smallwood and two graduate students record data for each bird: band number, weight, molting status. A plucked feather yields DNA. Studies have shown, he noted, that kestrels tolerate such handling without suffering nest losses. Meet some of the animals most affected as humans take over more and more land. The life history of the birds has yielded interesting observations, if not answers. Experienced kestrel parents are most likely to raise at least one surviving chick per nest, Dr. Smallwood has found, while the nests of first-time parents are most likely to fail. A young female and older male fare in the middle. So what about older females with young males? Females, the ones to choose the mate, dont seem to go for inexperienced partners. They know better, Dr. Smallwood said with a laugh. We havent seen it yet. Overall, the nest box research by Dr. Smallwood and other scientists indicates that the habitat looks good and the birds that show up are doing well. The problem is, he said, theyre just not showing up. Less is known about where migrating populations spend the winter or what happens there, though is trying to answer some of those questions. Scientists cant outfit kestrels with satellite trackers, as they did with the larger Swainsons hawks, because theyre too small. In a newly published special issue on kestrels in The Journal of Raptor Research, Dr. Smallwood and David Bird, an emeritus professor of wildlife biology at McGill University in Montreal, list seven possible factors for kestrel declines that they argue merit more research, in no particular order. Could a surge in the population of Coopers hawks be limiting kestrel habitat? Whats happening to kestrels winter habitat? In the spring, do agricultural fields lure kestrels to nest, only to let them down as the land changes over the season with planting or harvesting? Could kestrel declines be related to insect declines? Are rodenticides, a danger to all birds of prey who eat poisoned mice and rats, of special concern for kestrels? What are the effects of neonicotinoids, a particularly potent insecticide? What about the consequences of climate change? Many kestrel experts think its a combination of causes. Its just everything, said Jean-Francois Therrien, a senior scientist at Hawk Mountain, a conservation group for birds of prey. So many factors playing a small role, but adding up to the declines were seeing. Dr. Smallwood agrees, but he still has a top suspect. If Im only allowed one word: grasshoppers. Sure, kestrels also eat rodents and lizards. Dr. Smallwood is even seeing remains in nests that suggest theyre eating songbirds more than before. But he thinks a lack of insect prey is a major issue, a theory that may be bolstered by early results of an ambitious modeling effort that seeks to solve the mystery of declining kestrels once and for all. Funded by the United States Geological Survey and the U.S. Fish and Wildlife Service, the project is a partnership of more than 50 collaborators, including scientists from universities, conservation groups, states, Native American tribes and the federal government. Researchers are in the process of setting up and testing continent-scale models. The one parameter that seems to be declining over time, researchers say, is survival of young birds in the summer. Thats not a firm conclusion yet, by any means, because we havent finalized the modeling, said Brian Millsap, who recently retired as national raptor coordinator for the Fish and Wildlife Service and remains affiliated with New Mexico State University. But it seems like thats a finding that pops up no matter how you set the model up. Discussing the results with partners, he said, the thinking is that those juveniles may be more dependent on insect prey because its easier to catch. If that notion bears out, the next question is why the insects are declining. Pesticides? ? Entomologists are trying to figure that out, too. In the Anthropocene, one mystery leads to another. reports on biodiversity for the Climate and Environment desk. She has also worked on the Investigations desk, where she was part of the Times team that received the 2018 Pulitzer Prize for Public Service for its reporting on sexual harassment.