Call in the hawk

Protecting crops with predators instead of poisons

By Andy McGlashen for Environmental Health News

Hawks will take all kinds of small critters, including chipmunks.
(Photo by Will Collette)

Summer carloads of sweet-toothed tourists, flush with cash and seeking local pies and jams, are an economic godsend in northwest Michigan's cherry-growing region. 

Other hungry visitors are less welcome—voles, weevils, fruit flies, grasshoppers and pest birds do significant damage to local crops.

Cedar waxwings, American robins and other birds alone cost the state's tart and sweet cherry growers more than $4.3 million a year. To protect their bottom lines from nuisance birds, fruit farmers deploy a quirky arsenal.

Propane cannons frighten flocks (and neighbors) with epic blasts. Speakers blare recordings of bird distress calls. Balloons with menacing eyes loom overhead. But the clever birds soon learn these are empty threats. Their feast resumes.

Since the early 1990s, though, some local orchardists have had better success by enlisting natural helpers with real bite: American kestrels, small falcons that eagerly move in when farmers put up nest boxes and prey on a range of agricultural pests. 

For farmers, the predators provide an important service on the cheap. And for kestrels—North America's most widespread falcon, but a species whose numbers have plunged by nearly half in the past half-century—the setup provides a cozy home and ideal habitat.

Photo by Will Collette

Cherry growers aren't the only ones forming avian alliances. In a recent paper in the journal Agriculture, Ecosystems and Environment, Michigan State University scientists reviewed past research and concluded, farmers who add structures or manage their land to attract birds, bats and other vertebrates can boost profits, reduce pesticide use, and conserve vulnerable wildlife.

"There are species out there that, particularly when they live in agricultural landscapes, are providing services for us, and sometimes we're not even aware of them," said Catherine Lindell, a biologist and lead author of the paper. "The more we understand about these services they provide, the more we might be able to enhance those services by giving them the resources they need."

From New Zealand to California

Lindell and her co-authors note several examples. When scientists reintroduced a threatened, native falcon species to New Zealand vineyards, the raptors drove off 80 percent of non-native pest birds and reduced the crop damage they caused by 95 percent, saving some vintners more than $300 per hectare. Other researchers found European kestrels and barn owls helped control rodents on Spanish farms.

Raptors aren't the only birds with pest-control benefits: Adding nest boxes for great tits, a small songbird, cut caterpillar damage in half on apple orchards in the Netherlands; areas near bluebird boxes on a California vineyard had significantly fewer leaf-eating insects.

Other researchers are investigating ways to manage overall agricultural landscapes to attract beneficial birds. Studies show, for example, that weaving natural habitat likehedgerows into farmland can lure birds that gorge on harmful insects.

Since 2012, Lindell and graduate student Megan Shave have been working to better understand the role raptors can play in supporting Michigan's fruit industry. Preliminary results show pest birds are significantly less abundant in kestrel-guarded orchards. 

The avoided crop damage is enough to generate $2.2 million a year in additional revenue if all the state's sweet cherry growers added nest boxes.

Unlike balloons or loud noises, kestrels are a genuine threat whose mere presence is enough to scare off pest bird flocks, said cherry grower Jim Nugent, who installed a nest box on his roughly 40-acre Suttons Bay, Michigan, orchard in the mid-90s. It has been occupied almost every year since.

"Our problems with birds have really dropped off," he said. "They [kestrels] are quite effective, and it doesn't require much management on the grower's part."

Fewer pesticides and poisons

The idea isn't new; the U.S. Department of Agriculture established an "economic ornithology" unit in the 1880s to study birds for pest control. But that body was disbanded in 1940, around the time synthetic pesticides like DDT were hitting the market and being hailed as wonders for controlling insects that spread disease and ruined crops.

By the 1960s, scientists were beginning to understand the serious ecological effects of those chemical compounds. Rachel Carson's landmark 1962 book Silent Spring detailed how DDT built up in birds and made their egg shells too thin to protect chicks, and its title raised the prospect of a future deprived of birdsong. A decade later, DDT was banned, and populations of raptors and other birds began to rebound.

But other pesticides hit the market. Currently more than one billion pounds of pesticides are used in the U.S. each year, and more than 5.6 billion pounds are used globally. Scientists increasingly point to potential health hazards of this widespread use—as certain pesticides have been linked to genetic changes, cancers, endocrine disruption, nerve disorders, mental health issues and reproductive problems.

In response, some food producers and researchers have grown more interested in the on-farm benefits birds can provide.

At this point, it's not clear how much pesticide use could be offset by partnering with natural predators, said Lindell, who describes beneficial birds as "one tool in the toolkit" of an integrated pest management approach.

However, she and others caution that if farmers are drawing in predators, they should avoid toxics to control pests, since the poisons can work their way up the food chain and kill birds and other animals. 

A study by the state of California found that three-quarters of raptors, bobcats, coyotes and other wildlife tested positive for rodenticides.

"People love to think that we've got to have poison in the toolbox, and we say you can't have both," said Lisa Owens Viani, director of Raptors Are the Solution, a California-based project of the nonprofit Earth Island Institute that works to stop rodenticide use. 

"We think it's unfair to put up an owl box and lure an owl to a place where there's poison being used. The bottom line: Do we want them to help us control rodents, or do we want to poison them?"

Owens Viani said that, at least in some cases, raptors alone are enough to control rodents. She pointed to a recent study by California's Ventura County Watershed Protection District, which works to control burrowing rodents that can degrade levees and dams. 

In findings published last December, the agency reported that levee sections where workers had installed perches to attract raptors had substantially less damage from ground squirrels than areas treated with rodenticides. The report called for replacing the poisons with raptors system-wide, noting that the county would save $7,500 a year for each mile of levee.

In another California study, barn owls essentially formed a colony at a research site after nest boxes were installed, with the population at one point reaching 102 owls on a 100-acre vineyard. 

The birds killed more than 30,000 rodents over the course of three breeding seasons, for a fraction of the cost of trapping or poisoning them, said lead researcher Mark Browning, a biologist formerly with the Pittsburgh Zoo who now owns the Barn Owl Box Company, which sells the nest boxes.

That suggests, at least in some situations, natural predators could make rodenticides unnecessary, he said.

"The wonderful aspect of utilizing an animal such as the barn owl is that they do their work without any prodding," Browning said. "You don't have to wake them up in the morning or tell them they're not moving fast enough that day. And the pressure that barn owls exert on a population is unrelenting, particularly at the very time that rodents are building their numbers."

“There just isn’t a downside”

As a retired district horticulturalist for Michigan State's agricultural extension service, Nugent said kestrel nest boxes are one reason he's seen his fellow growers easing off of rodenticides to kill voles, mouse-like rodents that can kill fruit trees by gnawing on their bark and roots.

Migrating kestrels should return to Nugent's farm in a month or so, where they'll find fresh wood shavings he added when he cleaned out the nest box in the fall. 

That annual chore is pretty much the only upkeep needed to keep his winged workers on the job. "It's not a very big project," he said.

"There just isn't a downside."