Could the increase in Cooper’s hawk populations limit kestrel habitat? What is happening to the kestrel wintering grounds? Do agricultural fields attract kestrels to nest in the spring, only to release them when the soil is changed later in the season by planting or harvesting? Could the decline of kestrels be related to the decline of insects? Are rodenticides, a danger to all predators that eat poisoned mice and rats, special to kestrels? What are the effects of neonicotinoids, a particularly powerful 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, senior scientist at Hawk Mountain, a raptor conservation group. “So many factors play a small role, but along with the declines we’re seeing.”

Dr. Smallwood agrees, but still has a prime suspect.

“If I may only say one word: locusts.

A finding that keeps popping up

Of course, kestrels also eat rodents and lizards. Dr. Smallwood even sees remains in the nests that indicate they are eating warblers more than before. But he believes the main problem is a lack of insect prey, a theory that may be supported by the early results of ambitious modeling efforts to solve the mystery of the kestrels’ decline once and for all.

Funded by the United States Geological Survey and the US Fish and Wildlife Service, the project is a partnership of more than 50 collaborators, including scientists from universities, conservation groups, states, Indian tribes and the federal government. Researchers are in the process of creating and testing continent-scale models. The only parameter that appears to be decreasing over time, according to the researchers, is the survival of young in the summer.

“It’s by no means a firm conclusion because we haven’t finished the modeling,” 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 to be a finding that emerges no matter how you set up the model.”