Only a few years ago, American kestrels were a common sight in the Yukon.
The smallest of all North American falcons, the colourful birds could be seen perched on power lines and fences, or hovering in place as they scanned for prey, their narrow wings beating furiously.
Kestrels were so ubiquitous that even ardent bird-watchers tended to take them for granted, including Yukon biologist Dave Mossop.
“The little kestrel was everywhere,” says Mossop, an instructor at Yukon College. He regularly spotted as many as 50 of them along one stretch of the Alaska Highway. And he continually found them in places where he did not really want to see them — the nesting boxes he has installed throughout the Yukon’s boreal forest.
Mossop began putting up boxes to attract small owls, which typically nest in cavities in old growth trees. He and other researchers visit about 100 of the boxes every year, and for many years they found bright-eyed kestrels in about half of them, decreasing their data on owls, but offering a snapshot on Yukon kestrel populations that no one was looking for at first.
“But then all of a sudden the number of kestrels started to drop off, and two years ago it went to zero,” says Mossop. “I was shocked and started to cast about for information because I was not an expert in kestrels.”
He found that other kestrel researchers also had been noticing declines, but there had been little sharing of information, a crucial step to putting together a bigger picture on what was happening to the species. “That is the issue; biologists tend to go off into the field and work on their own little problems. But when a common bird starts to disappear, there is a lesson there. It is an indicator species; it is telling us something very serious.
For Mossop, there is a bit of deja vu going on. In the 1970s, when peregrine falcons almost disappeared from the continent, he was part of the massive North American effort to save the species by reintroducing captive-bred birds to the wild.
A little kestrel is a mid-morning snack for the mighty peregrine, and Mossop acknowledges that biologists are often tempted to study “bigger, more macho birds like peregrines and eagles.” But now that kestrels are in serious trouble, he hopes the same sort of networking that helped save peregrine falcons can be applied to kestrels.
DDT poisoning almost caused peregrines to go extinct, and while this pesticide also took its toll on kestrel populations in the 1970s, no one knows yet what is causing the dramatic recent declines. But researchers have begun to put their heads — and data — together.
Mossop and nine other scientists with nesting box programs spread across the continent co-authored a paper published in 2008. They had hoped that analysis of their shared information might pinpoint an obvious cause for the decline; while that did not happen, they did eliminate several possibilities, and confirm that kestrels are in trouble in many places.
One suspect was West Nile virus, which often infects kestrels as they feed on mosquitoes carrying the disease. However pathogens usually cause short sharp declines, followed by a rapid recovery. Kestrel populations have suffered a long steady decrease which began even before West Nile virus arrived in North America in 1999.
Cooper’s hawks prey on kestrels, and their numbers have been increasing in the northeastern United States, even as kestrel numbers have been declining there. However there are no Cooper’s hawks in some of the regions where kestrel populations have declined sharply.
Loss and degradation of habitat was also considered, though Mossop thinks that kestrels co-exist with humans quite well. If anything, he thinks that kestrels’ preference for insects could cause them grief. “If you eat insects, you are really in harm’s way for a lot of bad, bad chemicals,” he says.
Yukon kestrels probably wing their way as far south as the US Gulf Coast in winter, and possibly all the way to Mexico. Mossop is not sure because he seldom receives any reports on the kestrels he bands in summer, but he is sure that they go far enough to put them in harm’s way.
“I am convinced that it is something bigger than a regional problem. The idea that I am pursuing is that this is a small falcon, and they have to hunt all of the time when they are migrating. They are not like geese that can overfly country that has no food, and birds coming from the Yukon are covering some empty country.”
He thinks that kestrels could be an indicator species for “the biggest problem in the whole world — climate change. I have this niggling feeling that they would be very susceptible to changes in climate,” says Mossop.
While kestrels are clearly in trouble, he was relieved to find pairs of the small falcons in two nest boxes last summer. And he hopes that the new networks among kestrel researchers could help find the root problem, as it did decades ago with Peregrine Falcons.
For more information on the decline of the American kestrel, contact Dave Mossop at firstname.lastname@example.org.
This column is co-ordinated by the Northern Research Institute at Yukon College with financial support from Yukon Department of Environment and Yukon College. The articles are archived at