American Kestrel. Image: U.S. Fish & Wildlife Service
By Eric Freedman
New homes may help save a declining bird species and, at the same time, protect economically vital cherry crops from orchard-damaging enemies.
That’s the conclusion of scientists who placed nest boxes in Leelanau County, Michigan, cherry orchards in an effort to support more breeding populations of the American kestrel.
The kestrel – or sparrowhawk – is the smallest, most colorful and most common falcon in North America but faces “significant and widespread population declines,” according to the researchers. They describe it as “a species of conservation concern.”
The population of kestrels is declining about 1 percent a year nationally and in Michigan, said Rachelle Roake, the conservation science coordinator for the Michigan Audubon Society.
“They’re not doing that great,” although they’re not listed as a threatened or endangered species, Roake said.
Major factors in that decline include development that removes natural nesting cavities and snags, as well as climate change-related habitat loss on migration routes and in wintering grounds, according to researchers Catherine Lindell and Megan Shave of Michigan State University’s Department of Integrative Biology. They published their nest box findings in two new studies.
The shrinking number of kestrels is bad news for Michigan tart and sweet cherry growers whose crops are vulnerable to the grasshoppers, meadow voles and robins that kestrels like to chow down on. They also scare away robins, cedar waxwings and other fruit-loving birds, Lindell said.
Other crops, including apples, benefit as well from the presence of kestrels, she said. For example, voles eat the bark of young cherry and other fruit trees, killing them.
Nikki Rothwell, coordinator of MSU’s Northwest Michigan Horticulture Research Station near Traverse City, said it’s tough for cherry and grape growers to keep fruit-eating birds out. They’ve tried a variety of measures including balloons, sprays, nets and squawk boxes, all of which have major weaknesses.
The nest box project was “pretty neat” research,” Rothwell said. “It offers growers something they can do, something proactive.”
Lindell said sweet cherries are kestrels’ prime beneficiaries because they ripen at the same time as kestrels are nesting. Kestrels in Northern Michigan later migrate, usually to the southern United States.
Cherries are big business in Michigan, which leads the country in producing Montmorency tart cherries and ranks 4th in sweet cherry production, according to the Department of Agriculture and Rural Development. Overall, the state accounts for 70-75 percent of Montmorency tart cherries and 20 percent of sweet cherries production nationally.
As for filling the birds’ menu, the Cornell University Lab of Ornithology says, “Hunting for insects and other small prey in open territory, kestrels perch on wires or poles, or hover facing into the wind, flapping and adjusting their long tails to stay in place.”
The MSU scientists installed 23 nest boxes in 2012-2015 next to or within cherry orchards on the Old Mission and Leelanau peninsulas. Many were placed near the pastures, open fields and row crops where kestrels like to hunt.
The entrances faded southeastward to encourage kestrel occupancy and the survival of hatchlings.
The researchers monitored the boxes with pole-mounted cameras and opened the boxes to count eggs, hatchlings and fledglings.
Lindell said a similar nest box study is underway at blueberry farms in Western Michigan.
In Leelanau County, kestrels laid eggs in all 23 boxes and had “consistently high reproductive rates, indicating that the orchards and surrounding areas provide suitable habitat for successful kestrel breeding and fledgling production,” one of their new studies said.
“The results suggest that orchard nest boxes have the potential to sustain or increase the breeding kestrel population in the region while increasing kestrel predation of crop-damaging prey in and around cherry orchards,” the study in the Journal of Raptor Research said.
Their other study, published in the journal PLOS One, said, “Our results could encourage additional farmers to install and maintain nest boxes in fruit-growing regions where agricultural practices create open hunting habitat for kestrels.”
There were a few failures as well. Eggs in several boxes were abandoned because of competition from European starlings or another reason, and nestlings in a fourth box were killed by unknown assailants in a nighttime attack.
Meanwhile, cherry growers face other problems that kestrels can’t solve, according to Rothwell. For example, deer browse on trees, and bucks can kill trees by rubbing up against them. A fungal pathogen called cherry leaf spot can be devastating as well.