AgBioResearch

High atop a lone tree, a small, hawk-like bird with striking blue-gray, spotted plumage bobs its head and tail, then rockets into a field to strike its unsuspecting quarry. The American kestrel, North America’s smallest and most common bird of prey, can be found standing watch across the entire continent, from as far south as the Yucatan Peninsula to as far north as central Alaska. It is also common throughout much of Central and South America.

Across its extensive range, the diminutive raptor preys on a wide variety of small creatures, such as mice, voles, lizards and large insects such as grasshoppers and dragonflies, which it spots with its highly developed vision. This same superior sense of sight also allows them to intercept bats and small birds in midair. Not only has its varied diet staked out an important niche for the species in the food web – it also plays no small part in the birds’ historic success at maintaining high populations.

Today, however, these high populations are significantly reduced. Since 1966, estimates based on data collected by the U.S. Geological Survey, the National Audubon Society and the Raptor Population Index, indicate that American kestrels have experienced an approximately 47 percent decline across North America. In Michigan, their numbers have been reduced by 28 to 43 percent, depending on the region.

Many causes have been proposed for this decline, from competition with larger birds of prey to disease or habitat loss through development, but none has yet gained enough empirical support to fully explain the phenomenon.

Looking into the problem

The American kestrel is not without its allies, however. In 2011, a research team led by Michigan State University (MSU) integrative biologist Catherine Lindell received a four-year, approximately $2 million grant from the U.S. Department of Agriculture (USDA) Specialty Crops Research Initiative (SCRI) to investigate the impact of and possible solutions to crop damage by bird pests.

Word reached the team that blueberry growers in the western United States had set up nest boxes for the American kestrel to attract them to their lands and take advantage of the birds’ natural proclivity for hunting the insects, rodents and small birds that come to eat their crops.

“We thought it was something worth trying,” Lindell recalled. “We knew through MSU Extension that the cherry growers in Leelanau County were having similar problems with pest birds in their orchards, so it created an opportunity to test the technique in a region where it had the potential to really help people.”

Growers in Leelanau County had a sporadic history of setting up nest boxes for kestrels, often placing them atop the very telephone poles the birds themselves choose as their perches. Lindell wanted to approach the technique in a more systematic way that also brought the kestrels into the orchards proper.

For Megan Shave, it was an opportunity to blend her interests in conservation and agriculture. Joining the project in 2012 as a graduate student, Shave oversaw the installation of 10 nest boxes in orchards throughout Leelanau County, adding to the eight that were installed in early 2012. Shortly thereafter, every box was occupied by a nesting kestrel pair.

“I came to Catherine’s lab because I’ve always been interested in birds,” Shave said. “I wanted to not only continue studying bird behavior and population dynamics but also to look into how they relate to ecosystem services – how their presence could help human activity.”

Determining the value that kestrels have for their human counterparts is critical for sustaining long-term interest in their conservation, Shave said. She and her team set up video cameras in each nest box to record what types of prey the birds brought back home. At the same time, the team measured populations of pest birds in orchards with occupied nest boxes and compared them with populations in nest box-free orchards.

The preliminary results are optimistic. In surveys of identical sweet cherry plots, Shave and her colleagues recorded an average of 3.7 pest birds when no nest boxes were present; in those with nest boxes, that number plummeted to only 0.69. Though sweet cherries are much more susceptible to pest bird activity than their tart cherry cousins, the same trend was borne out there: plots without nest boxes saw an average of 1.33 pest birds; those with them reported an average of only 0.31. Shave attributes this not just to direct predation by kestrels but also to them having a deterrent effect.

“Kestrels are unlikely to eat all the birds in that area, but we found that just having an active kestrel box in an orchard serves as a warning to other birds in the area,” Shave said. “Kestrels and their young are very loud, and we think pests such as robins and starlings learn where raptors spend time and they avoid those spaces.”

Shave’s video cameras collected evidence that the kestrels were not only warding off pest birds but also preying on more terrestrial orchard pests.

Grasshoppers and other leafdamaging insects were frequent entrees in kestrel meals, as were rodents such as meadow voles, which often damage the root systems of young trees as they burrow.

In the years since the project began, the team has put more boxes throughout Leelanau County to continued success: approximately 90 percent of the boxes remain occupied, and the local kestrel population has seen significant improvement as a result.

The original SCRI project expired in 2015, but the team’s efforts have continued under a new three-year, $500,000 grant through the National Science Foundation’s Coupled Natural- Human Systems project.

“We’ve shown that when you put up nest boxes, more kestrels appear in the area,” Lindell said. “For example, last winter we put up five boxes along a route where there had been no kestrel sightings, and this season we’ve had reports of kestrels in the area.”

Working for growers

Ensuring that nest boxes help growers as well as kestrels is central to the project, and something that Phil Howard, associate professor in the MSU Department of Community Sustainability, is actively studying. A member of the team since the project began in 2011, Howard has focused on gauging the attitudes of consumers and growers alike with respect to kestrel conservation and ecosystem services.

“Depending on the crop, we found through surveys and focus groups that consumers were willing to pay 30 to 32 percent more for fruit from farms that practiced conservation through nest boxes,” Howard said. “Even if, in practice, it doesn’t quite garner that price premium, this shows there’s a lot of potential interest as consumers become more aware of nest box practices.”

With consumers interested, Howard and his graduate student, Chris Bardenhagen, who comes from a family of cherry growers in the Leelanau Peninsula, set out to work with growers, gauging their perspectives on the utility of nest boxes and their inhabitants for their orchards.

Through preliminary interviews with growers, Howard and Bardenhagen have identified a set of the 20 most significant factors – ranging from pest pressure to fruit quality to the cost of sprays and equipment – influencing grower decisions.

Bardenhagen’s task is to sit growers down with a 2-foot by 3-foot dry erase board with magnets representing each of the 20 factors. By having growers draw relationships between the factors – for example, higher fruit quality might be positively related to higher spray and equipment costs – Bardenhagen is able to gauge how growers perceive and consider the costs and benefits of orchard operations.

“Farmers do a lot of complex thinking when it comes to pest management,” Bardenhagen said. “They have numerous pests and diseases, different types of weather and combinations of conditions – it’s remarkably sophisticated. We’re helping them lay out their priorities in a visual manner, which gives us a better idea of what they’re most concerned about. And once they spend about five minutes on it, the farmers really enjoy the process.”

Howard hopes to use the information on how growers make complex decisions to inform a new online survey designed to deliver detailed information on kestrel nest boxes to growers and gauge their level of interest toward them as potential pest management solutions. That way, he says, the team can help connect growers with the resources they need to implement them.

Leelanau growers participating in the project are already reporting interest.

“We’ve gotten a lot of good feedback and cooperation from growers,” Shave said. “As we’ve expanded the project, new growers have been very interested and responsive about setting up boxes in their orchards. They tell me they often see kestrels flying over their property and believe the boxes are making a difference.”

Jim Nugent, president of the Michigan Tree Fruit Commission and owner of Sunblossom Orchards, a cherry orchard in Leelanau County, was an early adopter of kestrel nest boxes and has observed their benefits for many years.

“I’ve had a nesting pair of kestrels for many years now, and I just don’t have bird problems nearly as bad as I did before,” Nugent said. “I absolutely recommend them, especially for sweet cherry orchards. I always felt they were effective, but now, with the MSU study, we have data to back that up.”

More work ahead

The kestrel project is set to conclude in the summer of 2018, and though much has already been learned, Lindell, Shave, Howard, Bardenhagen and the rest of the team still have much more they hope to accomplish.

Chief among their remaining goals is finalizing the economic impact of using kestrels as a pest management tool. The team submitted their data to economists at the National Wildlife Research Center (NWRC), the research arm of the USDA Animal and Plant Health Inspection Service’s Wildlife Services program. The NWRC’s scientists focus on finding solutions to wildlife-caused damage across the spectrum of human activity, including agriculture.

In addition to measuring reductions in pest bird activity in orchards with active kestrel nest boxes, the team also estimated the volume of fruit removed from the orchards per minute by those birds.

“We were able to use that to figure out, when you have kestrels and thus fewer pest birds, how much fruit you could save with them in the field,” Lindell explained. “We’re in the process now of getting that data to our economist colleagues, who will translate that into actual financial savings.”

While the economic data are analyzed, the team plans to continue testing nest box strategies by expanding into new crops and regions of the state. Last year, Lindell and Shave traveled to the blueberry farms of Van Buren and Allegan counties in southwestern Michigan, where they worked with growers to set up new nest boxes.

To date, these efforts have seen a slower increase in local kestrel populations than in Leelanau County, which Lindell and Shave attribute to a number of potential factors. These counties feature more heavily wooded terrain as opposed to the more open environs in which the kestrel prefers to hunt. Forestland favors the much more common eastern starling, which could be outcompeting the kestrel for the nest boxes. Van Buren and Allegan also do not have the same history of farmers using nest boxes that the team found in Leelanau County, which Lindell and Shave believe contributes to having a lower starting kestrel population in the region. Despite these challenges, the team is already seeing an approximately 33 percent occupancy rate.

“Kestrels are migratory birds, and Michigan is right on the border of their migratory area,” Lindell said. “Looking at regions of the state that are farther south, closer to their year-round range, we might be able to make a larger impact in the long run.”

Conserving birds by helping growers
In 2012, the Peregrine Fund, a nonprofit organization working to improve the conservation of birds of prey around the world through research, launched the American Kestrel Partnership, a network of both professional and citizen scientists tasked with tracking and modeling kestrel populations around the entire Western Hemisphere. In so doing, the organization hopes to identify both the causes of kestrel decline and possible ways to reverse it.

As they explore the potential of kestrels as a pest management solution for fruit growers, Lindell’s team members have submitted their data to the American Kestrel Partnership to contribute to the conservation of the species.

“It’s not just for scientists,” Lindell noted. “Anyone interested in putting up a nest box can submit their data through an online tool.”

For growers interested in setting up nest boxes, the team has learned a number of lessons for getting the most out of them.

Placing them in open areas away from encroaching woodland and spacing them half a mile from other boxes significantly increase the odds that kestrels will occupy them by reducing competition for shelter from forest-dwelling starlings and for hunting ground from other kestrels. When looking for the ideal location, it is also important to note that kestrels have been observed to thrive the most in mixed landscape environments, featuring both young and old orchards accompanied by pasture land. The team also cautions against the use of rodenticides during the time period when kestrels are active in the landscape because the poison can be transferred to the birds through their prey.

“I love the fact that these birds are already out there, doing things that are helpful to us,” Lindell said. “If we can use nest boxes to better direct their activities and even enhance them, we can help them while they help us. We can help the birds, growers and, ultimately, consumers, all at the same time.”

Being able to conserve a struggling species and help improve the operations of Michigan growers at the same time makes this a dream project for a scientist like Shave.

“Kestrels are beautiful, charismatic animals that people are immediately intrigued by and care about,” Shave said. “This project has been a great example of how conservation and industry concerns don’t have to be mutually exclusive. We’ve shown how both sides can become invested in the goals of the other. It’s something that’s worthwhile for everyone involved.”