American Kestrel Population Drops Dramatically, And Without Fanfare

July 29, 2014

t’s late afternoon – an auspicious hunting time for birds of prey – and I’m leaning against a chain link fence watching the roofline of an abandoned building. Somewhere up there, I hope, an American kestrel will emerge from her nest.

Instead of a female emerging from the building, a male kestrel circles overhead, a small, determined shadow against the blue sky, before perching on a high branch of a nearby redwood tree. Through my binoculars I can see a hapless rodent dangling from his talons. Seconds later, a slight scrabbling noise draws my attention back to the building. I’m just in time to see the female kestrel slip out from under the eaves before she flies up to join her mate; the prey he’s delivering is a sure sign that eggs are in the nest.

An empty building in a residential neighborhood seems an unlikely place for kestrels to build a nest. The four-ounce falcons typically choose tree hollows to raise their young. But these birds of prey have a reputation for being adaptable. And finding the birds’ nesting site – anywhere – was encouraging for Zach Michelson and Teague Scott, volunteer researchers with the Santa Cruz Predatory Bird Research Group, who are investigating the local status of kestrels in response to reports that the falcon’s numbers are in a nationwide decline.

“No one has specifically studied the birds in this area. We want to lay the groundwork for repeatable surveys with our research,” said Michelson. “It’s only by following these kestrels year after year that we can find out if there are population changes, and if anything can be done to help.”

Zach Michelson (left) and Teague Scott, volunteer researchers with the Santa Cruz Predatory Bird Research Group, affiliated with UC Santa Cruz, watch for kestrels.
Zach Michelson (left) and Teague Scott, volunteer researchers with the Santa Cruz Predatory Bird Research Group, affiliated with UC Santa Cruz, watch for kestrels.

n a national level, the American kestrel (Falco sparverius) population has been plummeting. Records from the North American Breeding Bird Survey, a massive annual data collection effort for more than 400 bird species overseen by the U.S. Geological Survey (USGS) and the Canadian Wildlife Service, show the kestrels have declined by an estimated one and a half percent each year between 1966 and 2010. The long-term loss is almost 50 percent of the population. That’s a big drop for a bird considered abundant in North America.

Almost one third of the estimated four million birds in the global breeding population live in the United States, according to Partners in Flight, a nonprofit organization for landbird conservation. Some migrate seasonally, while others stay all year in areas with more hospitable climates, such as Santa Cruz. But over the decades the number of birds reported during breeding survey counts, hawk migrations, and the National Audubon Society’s Christmas Bird Count have dropped, occasionally precipitously.

In coastal California there’s been a 68 percent decrease across 106 routes monitored by the Breeding Bird Survey from 1966 to 2008. But the kestrels’ decline isn’t uniformly distributed across the country. For reasons that aren’t clear, the birds’ numbers are increasing around the central states and parts of Mexico, according to numbers gathered in the same survey.

A handful of things could be causing the lower kestrel numbers, bird biologists say, including increased predation by Cooper’s hawks, continued exposure to pesticides, and competition at nesting sites by European starlings. The land-clearing practices of “clean farming” may also get rid of the trees and brush that help make good places for the birds to nest and hunt. Even invasive plant species may be a factor if they grow too high on grasslands that once provided the short ground cover kestrels need to hover hunt, the birds’ unique style of flying in place over one hunting spot.

It’s just not easy to get a handle on kestrels.

“It’s hard to follow these cool little birds; they’re so fluid in their movement,” says kestrel researcher Elizabeth Wommack, a PhD graduate from the University of California at Berkeley’s department of integrative biology. Wommack, also a seven-plus year volunteer with the Golden Gate Raptor Observatory, has been studying kestrels across the continent to learn more about their population genetics. “The kestrels we’re seeing in California now could have been in Oregon or even Alaska. These birds are affected by weather and changes in their habitat, too.”

All the different ways of collecting information about them – breeding bird surveys, migratory bird counts, tracking birds with leg bands, nest box studies, or her own genetic studies – only gets researchers so far, she says. They all have information gaps. “The way I understand it, the Santa Cruz study sounds like it’s going to help fill in one of these gaps at the local level,” Wommack says.

american kestrel on light
A male kestrel perches on a light on the UC Santa Cruz campus. (Photo courtesy Zach Michelson)

ith my binoculars trained on the tall branches of the Redwood’s “feeding perch,” I see the female snatch the rodent away from her mate. Then she neatly dives down to the building. With a flash of russet feathers, she executes an aerobatic maneuver and tucks up under the roof ledge. The male remains on his perch for a few minutes, preening and preparing for his next hunting foray.

The field biologists have been watching this place for weeks. Before the birds chose this nesting spot, they followed pre-breeding rituals of kestrel behavior during which the male offered a selection of nesting sites. Once that choice was made, the birds picked a feeding perch where the male would bring prey to his mate before egg laying began. Breeding pairs can lay anywhere from four to seven eggs. Unlike other birds of prey, the kestrels take turns on nest duty. Incubation takes about 30 days. The hatchlings will be ready to fledge the nest another month later.

Michelson and Scott mapped out 12 study sites scattered throughout Santa Cruz County, based on historical sightings of birds in the area. They chose the most likely habitats to host kestrels using local information accumulated by the Santa Cruz Bird Club on breeding and wintering territories, and other suitable and accessible habitat patches. They also searched through virtual databases such as eBird and the Monterey Bird Board, as well as records accumulated by the Santa Cruz Bird Club, to choose the most likely habitats to host kestrels. “But there was no really long-term data to look at,” Michelson says. “So it wasn’t easy to put together a picture of the kestrel population here.”

In January and February they surveyed their selected spots for kestrels. By April, they had four breeding pairs in their sights – two more pairs than had last been noted in 2008. Then, in June and July, they returned to the same locations to follow the nesting process.

“We’ve been chasing birds all over the county,” Michelson says. In addition to the pair that nested in the vacant building — near Schwan Lake — they’ve also been following a breeding pair at the Moore Creek Preserve on the western end of town, and another set along the San Lorenzo River. The kestrels need about one square mile to have enough hunting available to support a nestful of hatchlings.

As they followed the birds, the biologists noticed these kestrels make do with the habitat at their disposal. The Moore Creek Preserve birds have an ideal location with acres of open grassland near their nest. But at the San Lorenzo site, close to the downtown area, the male kestrel has to commute; he arrives from several different directions each day to present insects, rodents, or the occasional snake, to his mate.

“Making huge flights to get prey may be how the birds are making up for lack of interconnectivity of their habitat,” Michelson says. When the birds live near people, their prey is more likely to be in fields some distance away.

But that adaptability may come at a cost. Studies show that closely fitting in to human habitats leads to higher stress levels in kestrels. Under more stress, more females abandon their nests too early resulting in young that don’t survive.

‘We can’t yet say if the Santa Cruz kestrel population is in trouble,” says Scott. “There could be lots of other breeding pairs where we aren’t surveying. That kind of analysis requires long-term data – maybe five or 10 years’ worth.”

female american kestrel
A female kestrel takes off from her favorite perch. (Photo by Teague Scott)

ven long-term data sets can be deceiving. The year-to-year fluctuations in commonly-found bird populations can mask critical turning points in species numbers, says Jessica Stanton, a USGS conservation biologist who spoke in early July at the North American Congress for Conservation Biology. So Stanton, along with USGS ecologist Wayne Thogmartin at the Upper Midwest Environmental Sciences center in Wisconsin, developed “risk metrics” to evaluate population numbers. These risk metrics forecast the probability of a species declining to 50 percent of its current abundance, or becoming rare within any given bird conservation region (BCR) in the near future. (The BCRs are defined by the North American Bird Conservation Initiative as “ecologically distinct regions in North America with similar bird communities, habitats, and resource management issues.”)

When Stanton applied the risk model to North American Breeding Bird Survey numbers for kestrels collected between the years of 1970 to 2012, she found a continent-wide declining trend across almost all of the bird conservation regions. In California, kestrel numbers decreased in 15 of in the 28 BCRs that Stanton analyzed.

Although Stanton says she wouldn’t call it a downward slide, and the results aren’t meant as hard predictions, the  risk analysis suggest that American kestrels could be in decline even though they’re not recognized as species of concern at regional or federal management levels . The next step is to go out and get data to try and see what’s happening.

“Our study is simple,” says Teague Scott. “But bird behavior is complex. We don’t make assumptions about what we see. We wait to see what the birds show us, and we do that repeatedly – and repeatably – over time.”

The three of us look up in time to catch the male taking off from the redwood tree. He makes one swift pass over the field next door before rapidly gaining altitude, beyond the range of our binoculars. He’s “specking out,” says Michelson, to places and things only kestrels know for sure.

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