California and Black-tailed Gnatcatchers at Oak Grove
From 31 August to 2 September 2021, Lori Hargrove and I completed our first round of bird surveys of the California Department of Fish and Wildlife’s Oak Grove Wildlife Area. Acquired a few years ago, the area includes much of the floor of Oak Grove Valley in north-central San Diego County, including all of the wash of Chihuahua Creek, and some of the surrounding slopes. Our most notable discovery: a new site for the California Gnatcatcher, at an elevation of 2800 to 3050 feet, higher than previously known. We observed two pairs along the wash of Chihuahua Creek and heard the species’ diagnostic calls at two other sites. I got a recording of the calls of one pair with my cell phone. So evidently there is a population, not just a wandering individual.
Not only does Oak Grove represent a new site for the California Gnatcatcher, it’s also a new site of sympatry of the California and Black-tailed Gnatcatchers. At two spots, the two species of gnatcatchers were in adjacent territories. Basically, what we found was an extension of the desert fauna that Ken Weaver described just 3 to 5 miles to the northwest at Aguanga in Riverside County and in Dameron Valley, straddling the county line (https://archive.westernfieldornithologists.org/archive/V42/WB-V42-1-Weaver.pdf). That includes, among the birds, also good numbers of the Cactus Wren (presumably the desert subspecies anthonyi), Ladder-backed Woodpecker (here overlapping with Nuttall’s Woodpecker), and Black-throated Sparrow. All these are resident on the coastal slope: Chihuahua Creek drains into Temecula Creek, which ultimately becomes the Santa Margarita River.
Among mammals, we noted several white-tailed antelope squirrels and among reptiles numerous juvenile zebra-tailed lizards, plus Lori saw one leopard lizard, all characteristically desert species also representing isolated populations here on the coastal slope. So we have already noted most of the species that make this area of unusual biogeographical interest. In his article, Ken Weaver called attention to the piecemeal development eroding the habitat supporting these isolated populations around Aguanga, so it is good news to find that a significant chunk of it has been conserved by CDFW.
We also noted the comparative sterility of the former agricultural fields on the valley floor now grown largely to mustard. Since we could not find a single California ground squirrel, we were discouraged over the possibility of the Burrowing Owl. Nevertheless, active rodent burrows were evident in some parts of the valley floor, so the search for the possibility of Stephens’ and Merriam’s kangaroo rats will be high priority for Scott Tremor when he makes his surveys of the area for mammals. The use of this habitat at other seasons is something to be investigated as well, but there is an opportunity for restoration of grassland, and some kind of restoration is part of CDFW’s eventual plan.
We have one 3-day survey per season scheduled for the upcoming year, so next spring and summer we’ll also focus on the question of persistence of the Gray Vireo, found in the surrounding area during field work for the San Diego County Bird Atlas 1997-2001.
A couple of weeks ago there was a little discussion of the fall migration of the Violet-green Swallow. On 1 September I saw 6 in Oak Grove Valley. Five were circling over the valley foraging so could represent postbreeding dispersal from nearby Palomar Mountain, but one emerged from the hills to the north and flew in a beeline to the south.
CDFW’s Oak Grove area is closed to the public, so our surveys through the San Diego Natural History Museum on behalf of CDFW represent a rare opportunity to explore this area to which we did not have good access for the bird atlas.