Flying With Kestrels

Atop a tall ladder, wildlife biologist Mark Martell peers into a nest box, hoping to find an American kestrel inside. He does and soon extracts the nesting female falcon, descending the ladder and holding her for other researchers to see. The bird is beautiful, a blue jay–size falcon with rusty tones, a gray crown, and a striped face, but the scientists determine that she’s too light to carry the tiny radio transmitter backpack they’d hoped to put on her. The bird flies away and the researchers go on to survey other nest boxes on a landscape of marsh, grassland, and woods on a May day at the Army Training Site in Arden Hills.

They’re looking for kestrels capable of carrying transmitters that will help track the bird’s life cycle. The data collected will help scientists understand what is behind the long decline in the American kestrel population. In Minnesota, the population declined by almost 78 percent from 1967 to 2019. 

Using automated radio telemetry—radio signals and receivers—to track the birds, researchers hope to gain enough information to deconstruct the life cycle of American kestrels that breed in Minnesota, the routes and timing of their migration journeys, and the biggest threats that occur along the way. Weighing less than 3 grams, the backpack transmitters send out signals that ping receiver towers along the birds’ migration route. The focus this year is to get transmitters on juveniles.

Kestrels at other Minnesota nesting sites are also getting the backpacks. The research can fill a knowledge gap, says Kristin Hall, a DNR biologist for the Nongame Wildlife Program who is leading the project. 

“We know quite a lot about their breeding behavior and their breeding needs, but we don’t know much about their migration or where they’re wintering. That’s why we put the transmitters on, to find out about additional annual life cycle travel,” Hall says. By the end of June they had placed 54 transmitters on kestrels over three years, with plans to place another six yet this season. 

The research is possible through the Minnesota Environment and Natural Resources Trust Fund and a multistate federal grant to expand the Motus tower network, whose receiver stations track migratory birds, including kestrels.

The biologists in Minnesota have already received data on birds that have migrated from the state and flown south through Iowa and Missouri. It is likely the birds are flying further to Texas, but they’ll know when the network is built out that far south. Researchers are already learning about survivability rates and migratory routes that weren’t clear before.

“What I really want to know is what are threats they face while on migration once they leave their breeding area,” Hall says.