Merced Vernal Pools and Grassland Reserve adds housing for a declining raptor
The sprawling grasslands of eastern Merced County should be American kestrel country. The area’s nearly treeless plains host clouds of grasshoppers and other arthropods as well as armies of voles, mice, and other small rodents. Grazing cattle keep vegetation short, helping kestrels see and swoop on prey. In other words, this corner of the San Joaquin Valley should be a veritable cornucopia for North America’s smallest raptor.
Even so, numbers of this colorful raptor have fallen in the region over the past few years. This downward trend is mirrored across much of the nation. According to the North American Breeding Bird Survey, almost half of the continent’s kestrels disappeared between 1966 to 2010.
Merced Vernal Pools and Grassland Reserve is doing its part to aid local kestrels while introducing UC Merced students to conservation research. This past spring, the reserve built and erected ten nest boxes to bolster the bird’s breeding populations.
“The overall goal was to expose students to the lifestyle of a fascinating small falcon and at the same time participate in a conservation project,” says reserve director and ornithologist Chris Swarth.
The first conservation project
The kestrel breeding project gave UC Merced students the chance to pitch in at every level from constructing the boxes to writing up their results in a report. It also represents the first conservation project to be completed at the reserve since it joined the UC Natural Reserve System this past January. The 39th reserve in the system, Merced Vernal Pools and Grasslands added 6,561 acres to the over 750,000 acres in the world’s largest university-administered reserve system.
A local nesting crunch nudged the reserve into the kestrel housing business. After years of hosting nesting kestrels, a neighboring rancher decided that more than 100 nest boxes on their land had to go.
Reserve director Swarth heard about the eviction, and realized the reserve could help fill the void. At the same time, he puzzled over the lack of kestrel families despite seeing adult birds on a regular basis. No pair has been recorded breeding on the reserve.
No cavities, no kestrel nests
“I think there used to be more trees in the region, willows and cottonwoods, like around the campus, and the creek channels out of the foothills. Kestrels require a hole in a tree or a man-made box for their nest. Over time, trees have been lost with the conversion of oak woodlands and riparian areas to agriculture and development,” Swarth says.
The loss of trees also means loss of the habitat kestrels rely on to raise a family. Rather than constructing their own homesteads by weaving sticks together, kestrels have long relied on tree cavities excavated by woodpeckers to shelter their helpless chicks. This penchant for prefab tree hollows has hurt kestrels, wood ducks, barn owls, bluebirds, and dozens of other cavity-nesting birds unable to drill their own abodes.
Conservation-minded humans are now stepping in to fill the void. These include retired Merced High School woodshop teacher Steve Simmons. Simmons has built and monitored thousands of nest boxes over the years for wood ducks, barn owls, and kestrels, and has banded 76,000 wood ducks over the past 41 years—more than anyone else in North America.
“Steve really is the best example of the tradition of a citizen scientist,” Swarth says.
Keeping tabs on nesters
Swarth enlisted Simmons to get the reserve nest box program off the ground. Seven undergraduates who had completed naturalist training at the reserve volunteered to monitor the boxes.
In spring, Simmons invited the students to his house to construct ten kestrel boxes from scratch. The team then fastened the boxes to existing fence posts around the reserve. From May to June, the students began checking on the nests. They counted eggs, chicks, and parent birds, and placed U.S. Fish and Wildlife Service bands on all birds old enough to sport ankle bracelets.
“Every time the students saw chicks or an adult bird inside they got very excited. It was the first time any had handled a wild bird, and they got to observe many up close.” Swarth said.
The students also engaged in forensic sleuthing to learn what was on kestrel menus. Throughout the nesting season, the team searched beneath the nest boxes for pellets of indigestible food regurgitated by the kestrels after meals. Back on campus, they compared the contents of the pellets to a reference collection of insects found on the reserve.
“As you pull out different exoskeletal parts from the pellets—femurs, head capsules, wings—you match them to insects in the collection, determining which species it belongs to and how many individuals are in a pellet,” Swarth says. “This gives you an indication of their relative contribution to the diet of the kestrel. Are they eating more crickets? Grasshopper species A or B? Or beetles?” The few mammal bones they found included a vole jaw and four pocket gopher skulls.
By season’s end, the team found six kestrel pairs had settled into reserve boxes. Some eggs disappeared, suggesting they were swallowed by gopher snakes. But four of the couples fledged a total of 15 young, with two pairs raising an impressive five chicks apiece. Additional details about this year’s kestrel breeding program are summarized in a report by Swarth, Simmons, and reserve intern Cami Vega.
A good place to raise a family
Reserve birds did well compared to kestrels nesting elsewhere in Merced County, according to Simmons. Drought, predation, and eviction by non-native starlings dogged many of the kestrel families in the 151 other boxes he monitored.
Swarth plans to expand the reserve’s nest box program in the future, offering Merced students and staff the chance to gain more insights into the lives of their fierce, feathered neighbors.
“I think everybody who’s worked on it is excited and has a feeling of anticipation about how things are going to go next year,” says Swarth. “Will we recapture a chick who is now a breeding adult? Will banded parents return to their same boxes next spring?”
Only time and next year’s nest box program will tell.