American Kestrels Are in a Puzzling Decline

In a newly published special issue on kestrels in The Journal of Raptor Research, Dr. Smallwood and David Bird, an emeritus professor of wildlife biology at McGill University in Montreal, list seven possible factors for kestrel declines that they argue merit more research, in no particular order.

Could a surge in the population of Cooper’s hawks be limiting kestrel habitat? What’s happening to kestrels’ winter habitat? In the spring, do agricultural fields lure kestrels to nest, only to let them down as the land changes over the season with planting or harvesting? Could kestrel declines be related to insect declines? Are rodenticides, a danger to all birds of prey who eat poisoned mice and rats, of special concern for kestrels? What are the effects of neonicotinoids, a particularly potent insecticide? What about the consequences of climate change?

Many kestrel experts think it’s a combination of causes.

“It’s just everything,” said Jean-Francois Therrien, a senior scientist at Hawk Mountain, a conservation group for birds of prey. “So many factors playing a small role, but adding up to the declines we’re seeing.”

Dr. Smallwood agrees, but he still has a top suspect.

“If I’m only allowed one word: grasshoppers.”

Sure, kestrels also eat rodents and lizards. Dr. Smallwood is even seeing remains in nests that suggest they’re eating songbirds more than before. But he thinks a lack of insect prey is a major issue, a theory that may be bolstered by early results of an ambitious modeling effort that seeks to solve the mystery of declining kestrels once and for all.

Funded by the United States Geological Survey and the U.S. Fish and Wildlife Service, the project is a partnership of more than 50 collaborators, including scientists from universities, conservation groups, states, Native American tribes and the federal government. Researchers are in the process of setting up and testing continent-scale models. The one parameter that seems to be declining over time, researchers say, is survival of young birds in the summer.

“That’s not a firm conclusion yet, by any means, because we haven’t finalized the modeling,” said Brian Millsap, who recently retired as national raptor coordinator for the Fish and Wildlife Service and remains affiliated with New Mexico State University. “But it seems like that’s a finding that pops up no matter how you set the model up.”

Discussing the results with partners, he said, the thinking is that those juveniles may be more dependent on insect prey because it’s easier to catch.

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