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John Smallwood is crazy about American Kestrels. Known as the “” to his colleagues, the professor of biology at Montclair State University has been studying North America’s smallest falcon for nearly four decades. His research has helped answer many questions about the species, from which prey they prefer to why they make certain habitat choices. But now he’s facing a new kestrel enigma: Why are the falcon’s populations declining year after year?

According to the North American Breeding Bird Survey, there are about 4 million American Kestrels living in the Lower 48, Mexico, and nearly all of Canada and Alaska. That’s 66 percent fewer kestrels than there were in 1966.

When Smallwood first picked up on this trend, he was practicing his carpentry skills—to help kestrels, of course. “We noticed in the early 2000s that [the species] was declining, when each year we found fewer breeding pairs in our nest boxes,” he says. He’s built, installed, and monitored hundreds of these artificial homes in Florida and New Jersey since the ‘90s. Based on the number of couples that have moved in, plus information gathered from breeding bird surveys, Smallwood has calculated a 4 percent decline in kestrel populations in Florida and a 5 percent decline in New Jersey.

Over time, Smallwood has settled on two hypotheses for the slump—the more obvious one being habitat loss. Like most falcons, kestrels are attracted to areas with open habitats, like fallow fields, which allow for easy hunting. But they’re also cavity nesters, preferring tree hollows and urban crevices, such as cracked brick facades and flat ledges.

During the ‘80s and ‘90s, many of the natural fields and forests kestrels called home were cleared and developed, resulting in fewer natural nesting spots for the birds. That’s where the nest boxes come in. With their pitched roofs and woodchip linings, they serve as the perfect stand-in for a typical kestrel home. But Smallwood’s records show that the raptors are also disappearing from areas where boxes have been installed. So if nesting spots aren’t the problem, what is?

Smallwood—for one—suspects pesticides. Over the last 50 years, biocides have been implicated in die-offs in Ospreys, Peregrine Falcons, and Bald Eagles. Most of those species have since recovered, thanks to bans and limits on DDT and endrin. But Smallwood’s research shows that modern-day pesticides—particularly the weed killer paraquat and neonicotinoids—may still be in small-bodied raptors such as kestrels.

But pesticides alone don’t complete the equation, says David Bird, professor emeritus at McGill University in Montreal and another longtime kestrel researcher. “It’s not like kestrel eggs are failing to hatch, which would be indicative of a toxicological problem,” he says. “We need to look further at something pesticides might be affecting, and that’s kestrels’ food source.”

While kestrels do feed on small rodents and birds, most of their diet is made up of large insects like grasshoppers and earthworms. Bird says pesticides may be making it more difficult for kestrels to find their preferred food source. Indeed, suggest that insecticides—particularly neonics—have negative consequences that travel up the food chain, affecting a wide range of avians.

“When you look at the big picture of other insect-eating birds out there—swifts, swallows, Purple Martins, Loggerhead Shrikes, whippoorwills, nighthawks—many are declining,” says Bird. “A loss of birds is the price we have to pay for the war we’ve been waging on insects for decades.”

On top of habitat loss and food-supply pressures, kestrels with diseases like West Nile virus and predation by Cooper’s Hawks. They also have to compete for nest space with invasive species, such as European Starlings. The invasive songbirds are notorious for displacing native cavity nesters, and can be aggressive when it comes to taking over kestrel boxes, says Juliane Wohler, a board member with the in New York. She has been monitoring boxes throughout Long Island for six years.

Wohler confirms that kestrel populations in Long Island have dropped significantly over the past several decades, too. Rampant development and pesticide use on farms and suburban lawns in the area could be the root of the cause.

The good news is, the downward trend in kestrel populations may finally be leveling off. “Their decline has now slowed in most areas,” Smallwood says. “But figuring out what’s causing current losses can help guide conservation strategies going forward.”

For the time being, Smallwood and other experts will continue to raise the alarm and keep a close eye on the species. But kestrel lovers and bird enthusiasts can help as well, by volunteering to build and check nest boxes in their area. (Some local ̽����ѡ chapters, such as Great South Bay, run community-wide monitoring programs.) Take it from Wohler: “Observing these majestic birds is our best chance of preventing further losses," she says. "Plus, they’re just so much fun to watch.”