from the Field
Gray Vireos Discovered Wintering in Elephant Trees, or, Isn't Science Grand?
Back on 4 May 1999, Lori Hargrove made the long hike to cover square K28, about 2.5 miles from the Elephant Trees trailhead in Anza-Borrego Desert State Park, off Split Mountain Road, just to get to the edge of the square. Lori noticed the large stands of Elephant Trees in K28 along the canyon leading to Starfish Cove and mentioned them to me. This reminded me of a paper I had read in the Southwestern Naturalist in 1992, by John Bates, now at the Field Museum of Natural History in Chicago. The subject of John's master's thesis at the University of Arizona was the Gray Vireo's wintering ecology, a previously unstudied topic. In Sonora, the Gray Vireo's main winter range, he found a mutualistic relationship between the Gray Vireo and the Elephant Tree: the vireo feeds mainly on the tree's fruit, while the tree relies on the vireo to disperse its seeds, through regurgitation after the bird has digested the pulp.
Even though the Gray Vireo had never been confirmed wintering in California, I reasoned that a substantial population of Elephant Trees must need some seed-dispersers. So the stand of Elephant Trees in the Starfish Cove areaconfirmed as the largest in the state after a survey by helicopter on 21 January 2000 by botanist Jim Diceshould the best place to look. As you may recall from the last issue of Wrenderings, I put out a call for participants interested in mounting an expedition to square K28, on 4 and 5 December. To ensure us enough time to search the area well at a time of year when the days are short, I figured two days were necessary, requiring backpacking. Maryanne Bache, Lori Hargrove, Mark Hoefer, Ginger Rebstock, Bob Sanger, Jack Schlotte, Dave Seals, and Jim Zimmer answered the call, and with a group of this size we were able to split up and cover more areas, some repeatedly. In essence, it was a mini blockbuster weekend to squares K28 and K29.
We started up Alma Wash at sunrise, hiking briskly at first to make it to the concentrations of Elephant Trees as early as possible. At 8:15 we stopped briefly because of a small flock of birdsin this desert where birds are so sparse, every one needs to be looked at. A small gray bird caught our eye, and, suspicious of the possibility, I asked Dave to break out his tape recorder and play the tape that John Bates had lent me for the expedition. The bird was interested and came into viewour first Gray Vireo! It didn't sing in response but uttered a short sharp trill, a call unlike that of any other vireo in California.
Continuing on, we reached our campsite at the mouth of the canyon, on the line between squares K28 and K29. After leaving our packs, we split up, Bob and Jim scouting the steep slopes, the rest of us continuing on the remaining two miles to Starfish Cove. This hike revealed two more Gray Vireos, in thickets of the Desert Lavender mixed with Elephant Trees. Interestingly, we found both birds visually, though one sang in response to our playing the tape after we sighted it. The other scolded us with a call much like the one Hutton's Vireo uses occasionally.
The next morning we divided into three groups. Bob and Jim headed up the canyon to Starfish Cove, retracing the route of the rest of the group the day before, and again finding two Gray Vireos, not far from where they had been on Saturday. Maryanne, Lori, and Ginger followed the base of the steep mountains about three quarters of a mile to the south to the next canyon. Here they found another Gray Vireo, singing spontaneously. Mark, Jack, Dave, and I circulated over the northern half of the bajada labeled "Elephant Tree Area" on the topo map. Unfortunately, the Elephant Trees are actually very sparse in this area, and we didn't find any vireos there.
We couldn't get any photos but saw all the vireos extremely well, noting their size slightly smaller than a Black-throated Sparrow, bill short and thick for a vireo, uniformly gray upperparts, whitish underparts, complete narrow white eye ring, lack of white lores, single narrow white wing bar, and narrow white edges on the secondaries. We got great coverage of both squares K28 and K29, in K28 recording 23 species, one over the square's winter threshold number of 22. Camping overnight helpedLori heard a Poor-will, Dave a Great Horned Owl in the predawn darkness. Rufous-crowned Sparrows, seen independently by two groups, were also a notable discovery, implying that species occurs all around the Vallecito Mountains, where they were not known previously.
Lori made two other visits to the area, covering just the wash on the bajada, below the narrow canyon enclosing Starfish Cove. She found at least one Gray Vireo that we didn't see on the weekend expedition, making a minimum of five vireos in the area. There could be a few more we didn't locate; many of the slopes with Elephant Trees are extremely steep and unstable. But our finding the birds so readily on a first attempt implies they are a regular winter visitor to the area, adding a new species to San Diego County's and California's winter avifauna.
The Gray Vireo has long been recognized as a seriously declining species in California, owing to brood parasitism by the Brown-headed Cowbird. The Elephant Tree itself is considered endangered in California by the California Native Plant Society. Quite possibly, the Gray Vireos wintering in California, at the northwestern corner of the winter range, breed in California, the northwestern corner of the breeding range. The viability of the Elephant Tree in California could be affected by an interaction between two birds that takes place far from the trees themselves!
It isn't often in field biology that you have a chance to develop a hypothesis that is so readily testable. In science, when experimentation and observation confirm prediction it's time to celebrate. Thanks very much to all the participants in the weekend for making it so successful. And we must again thank CalTrans and Pam Beare for the funding under which I sent Lori to Starfish Cove last May, the catalyst that set the whole process in motion. On a map, square K28 looks like the most remote and desolate square imaginable, and in a sense it is. Yet it yielded one of the most notable results of the entire atlas effort so far, a stunning example of why thorough coverageexploring the unknownis worthwhile.
Adapters versus Nonadapters
Last year, Barbara Kus of the Biological Resources Division of the U.S. Geological Survey invited me to speak about the San Diego County Bird Atlas to the Wildlife Society at its meeting in Riverside in January. This was a great opportunity to introduce the project to the broader community of professional wildlife biologists and managers that could be among the primary users of our results, so I accepted Barbara's invitation eagerly. I chose as a topic one I hoped would be relevant to a group familiar with San Diego's multiple-species conservation plans or similar plans in other areas: adaptationor failure to adaptto urbanization. Our atlas results are ideal for illustrating the effects of habitat fragmentationa hot topic in conservation ecology. Project Wildlife invited me to speak at its monthly meeting, and I gave a similar presentation there. The theme of adapters versus nonadapters with respect to urbanization will be a central one in the atlas and one I'll explore more in future issues of Wrenderings.