Environment / Nature

Large communal roost of Swainson’s hawks discovered in Panoche Valley

At least 143 individuals are documented.
37 Swainson's Hawks amongst the Common Ravens. They are believed to be part of a flock of 107 which were found at Panoche Valley. Photo courtesy of Steve Rottenborn.
37 Swainson's Hawks amongst the Common Ravens. They are believed to be part of a flock of 107 which were found at Panoche Valley. Photo courtesy of Steve Rottenborn.

Hollister resident, birder and naturalist Debi Shearwater noted a rare find in San Benito County: Swainson’s Hawks (Buteo swainsoni)—at least 143 of them. 

Shearwater, who has led bird tours all over the world, said she had not expected to find so many of the species in Panoche Valley as the bird is rare in San Benito County, though common in the Central Valley.

“The first assumed nesting of these hawks in our county was only discovered in 2013,” she said. “The number of nesting pairs exploded last year.”

Shearwater monitored two nests for the 2021 season—one along Lone Tree Road and the other in Panoche Valley. The Lone Tree Road pair had two young leave the nest successfully. She said the Panoche Valley nest was predated by a great-horned owl which likely fed the nestlings to their own young. 

She said this is the first historical record of such a large flock in the county. It is the first flock found west of the normal range. 

On May 30, on a trip to Panoche Valley, Shearwater counted 143 Swainson’s and on June 6 she returned there after hearing from a local rancher that some hawks were still in the area. She found 12 birds on his ranch. 

It was a matter of chance and the misfortune of her car dying that led to her next sighting on June 13. After her car died on New Idria Road, she and the tow truck driver, Alex, discovered 74 Swainson’s coming into a communal roost site in a grove of eucalyptus on a private ranch at 8:18 p.m. On June 18 she counted 48 individuals. 

On June 25, after pressing four friends, Steve & Heather Rottenborn, Clay Kempf and Pamela King, to join her in Panoche Valley, Shearwater spotted hawks, all standing on the ground in a small valley. Using a scope they counted 107 Swainson’s hawks. 

She said the flock seemed to be mostly non-breeding subadults. 

Shearwater told BenitoLink that this species feeds mostly on grasshoppers and that the area experienced a large grasshopper outbreak this summer. She said one hawk can eat up to 100 grasshoppers a day. Chris Hauser, manager of the Panoche Valley Preserve, told her there were millions of grasshoppers on the preserve this year. 

According to the Cornell Lab of Ornithology’s website “All About Birds,” the Swainson’s hawk soars on narrow wings and in flight wings are held in a shallow “V” (dihedral) shape. They prey on grasshoppers, small mammals and reptiles. The North American population migrates to Argentina for the winter. 

This hawk, 19-22 inches in length and weighing 24-48 ounces, is smaller than the red-tailed hawk and larger than the red-shouldered hawk, both of which are common throughout San Benito County. It has a wingspan of 46–54 inches. Its preferred habitat is grasslands, though it will nest in a variety of flatlands. Clutch size is 1-5 eggs which are incubated for about 35 days and young fledge at 40-45 days. 

For more information on Swainson’s hawks visit here. 

 

We need your help. Support local, independent news. BenitoLink is a nonprofit news website that reports on San Benito County. Our team is committed to this community and providing essential, accurate information to our fellow residents. It is expensive to produce local news and community support is what keeps the news flowing. Please consider supporting BenitoLink, San Benito County’s news.

Carmel de Bertaut

Carmel has a BA in Natural Sciences/Biodiversity Stewardship from San Jose State University and an AA in Communications Studies from West Valley Community College. She reports on science and the environment, arts and human interest pieces.Carmel has worked in the ecological and communication fields and is an avid creative writer and hiker.She has been reporting for BenitoLink since May, 2018 and covers Science and the Environment and Arts and Culture.