The Mystery of the Disappearing Kestrels: What’s Happening to This Striking Falcon?

Researchers are concerned about the decline in the population of kestrels, a flashy falcon that boasts of their reputation as predator birds. Despite their small size, kestrels are formidable hunters that prey on rodents, lizards, and insects, and they may eat small birds. In a recently published issue about kestrels in The Journal of Raptor Research, Dr. Smallwood and David Bird, an emeritus professor of wildlife biology at McGill University in Montreal, have compiled a list of seven possible factors behind kestrel declines that they believe merit more research.

They are arguing that a surge in the population of Cooper’s hawks may be limiting kestrel habitat. The researchers are also raising questions such as what’s happening to the winter habitat of kestrels. In the spring, are the agricultural fields attracting the kestrels to nest, only to let them down as the land changes over the season with planting or harvesting? The researchers are also inquiring as to whether kestrel declines are related to insect declines. Are rodents, which are a danger to all birds of prey that eat poisoned mice and rats, posing special risks for kestrels? What are the effects of neonicotinoids, a particularly potent insecticide? What about the consequences of climate change?

Many experts on kestrels believe it’s a combination of various factors. “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.”

However, Dr. Smallwood still suspects that grasshoppers are the primary reason for the decline in the population of kestrels. While kestrels eat rodents and lizards, Dr. Smallwood has found remains in the nests that suggest they are eating more songbirds than before. But he believes a lack of insect prey is a major issue, a theory that may be supported by early results of an ambitious modeling effort that aims to solve the mystery of declining kestrels once and for all.

The US Geological Survey and the US Fish and Wildlife Service are funding the project, which involves over 50 partners, including scientists from universities, conservation groups, states, Native American tribes, and the federal government. The researchers are currently setting up and testing continent-scale models. According to the researchers, the one parameter that seems to be decreasing over time is the survival of young birds in the summer.

Brian Millsap, who retired recently as the national raptor coordinator for the Fish and Wildlife Service and remains associated with New Mexico State University, said that the results are not conclusive yet because the modeling is not yet finalized. But, he added, that finding keeps popping up, no matter how they set the model up. He suggests that juvenile kestrels may be more reliant on insect prey since it is easier to capture.