The mystery of the disappearing kestrels: what happens to this flashy falcon?

Could an increase in the population of Cooper’s hawks limit kestrel habitat? What happens to the winter quarters of kestrels? Do farmlands run kestrels to nest in the spring, only to abandon them as the land changes during the course of the season with planting or harvesting? Could the decline of the kestrel be related to the decline of insects? Are rodenticides, a danger to all raptors that eat poisoned mice and rats, a special concern for kestrels? What are the effects of neonicotinoids, a very powerful insecticide? What about the effects of climate change?

Many kestrel experts believe it is a combination of causes.

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

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

“If I may only have one word: locusts.”

A finding that keeps popping up

Sure, kestrels also eat rodents and lizards. Dr. Smallwood even sees leftovers in nests that suggest they are eating songbirds more than before. But he thinks a lack of insect prey is a big problem, a theory that could be supported by early results of an ambitious modeling effort that is trying to solve the mystery of the descending kestrels once and for all.

The project, funded by the United States Geological Survey and the US Fish and Wildlife Service, is a collaboration of more than 50 collaborators, including scientists from universities, conservation organizations, states, Native American tribes and the federal government. Researchers are setting up and testing continent-scale models. The only parameter that seems to decrease over time, researchers say, is the survival of fledglings through the summer.

“That’s not a definitive conclusion by any means, because we haven’t completed the modeling yet,” said Brian Millsap, who recently retired as the 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 up the model.”