San Diego Natural History Museum--Your Nature Connection[San Diego County Bird Atlas Project]

WRENDERINGS
Reports from the Field
Spring 2002

Return to Starfish Cove

Sketch of Gray Vireo by Nicole Perretta

Gray Vireo

On 16 and 17 February 2002, a small group of us made the repeat trek to the Elephant Tree Area (K29) and Starfish Cove (K28) in search of wintering Gray Vireos. Two years ago, Phil Unitt organized the first search party after I told him about the large numbers of Elephant Trees in this area and he remembered research by John Bates that showed a strong association between wintering Gray Vireos and Elephant Trees (Bursera microphylla) in Mexico. Sure enough, in December 1999, we found about five individual birds widely scattered in the Elephant Tree habitat--a first record for wintering Gray Vireos in California and strong support for Bates' research. That year I made two other visits, finding birds in similar locations. The next year I made a repeat visit in November 2000, but found no Gray Vireos. On a third year's visit, in October 2001, I found only one Gray Vireo. And on this trip, despite good coverage of the area by our group, no Gray Vireos were found.

In winter, Elephant Trees may bear thousands of small (1/4 inch) reddish berrylike fruits. The winter range of the Gray Vireos coincides strongly with the range of the Elephant Trees, and the fruits are an important component of the Gray Vireo's winter diet. The large gape of the Gray Vireo allows it to swallow the fruit whole, then regurgitate the seeds later, dispersing them. Other species that feed on the berries do not necessarily disperse the seeds and do not show the same range matching. Thus, there may be a mutual dependence between these two species, and the possibility of coevolution. The Gray Vireos spend the breeding season a short distance to the north, patchily distributed across the Southwest. Our San Diego Bird Atlas found only a few dozen pairs in extensive areas of arid chaparral, yet this is likely the largest population in all of California.

The Elephant Tree Area is a large rocky alluvial fan on the eastern end of the Vallecito Mountains, in Anza-Borrego Desert State Park. There is a small nature trail with one or two Elephant Trees, but it takes a 3-mile hike west across the alluvial fan and its myriad of washes to see good numbers, and then there are even more in a canyon that leads 2-3 miles farther west to Starfish Cove (so named by Jerry Schad because of its starfish shape on topo maps--read his "Afoot and Afield in San Diego County" for a better description of the hike). This area has the largest stand of Elephant Trees in California, with only a few very small patches elsewhere in the Anza-Borrego Desert. The trees grow no more than about 8 feet tall and have amazingly twisted elephantine trunks. The smaller branchlets are the same maroon color as the surrounding varnished rocks, and when leafed out, the trees stand out with their bright green foliage.

On this trip, we backpacked across the alluvial fan to the canyon mouth where we spent the night. The winds sounded so ferocious that I began to wonder if the rocks were about to start flying around. But come dawn it wasn't too bad, and we split up for most of the day. Jim Determan, Bob Sanger, and Kathy Williams made the hike to Starfish Cove and covered the central portion of the alluvial fan. Phil Unitt hiked to another canyon to the south and covered the southern portion of the alluvial fan. I covered part of the main canyon and the northern half of the alluvial fan. Black-tailed Gnatcatchers were about the closest thing to a Gray Vireo we could find, similarly pale gray and flicking their tails while foraging. I stopped often to play a Gray Vireo tape, and listened for their song and distinctive trilling call, both of which I have heard on previous trips. Despite not finding any Gray Vireos, and numbers of most other species being much lower than two years ago, we ended the trip with a list of 23 species, decent for a waterless area in a drought year. Including previous years' makes for a combined total for the two squares of 35 species in winter, relatively diverse for this desert region.

So why the apparent lack of Gray Vireos in the second and third years? A very likely factor seems to be the amount of fruit. On the first year, many of the trees were covered with fruit. The second year, there was almost no fruit, and this year, the amount was moderate to scanty. Perhaps the birds continue farther south if there isn't a sufficient amount of fruit? It would be interesting to find a measure of what an average amount of fruit is for any one year in the area, and see if there is a correlation between amount of fruit and numbers of Gray Vireos. The amount of fruiting could be determined by several factors, including temperature and precipitation. Yet, perhaps the weather itself directly affects the Gray Vireos, causing them to choose to migrate farther south in some years. Or, since this is the northernmost extension of their range, if the overall population of Gray Vireos is down, the range might retract south out of this area.

This is one of the great qualities of our Bird Atlas project--while it is producing wonderful and informative data, it is also helping to create even more questions for future research. I'm sure we've all come to favor certain species or places, and scratched our heads in puzzlement with some of our observations. Though the Atlas is now ended, we can follow up with fieldwork that focuses on these questions. (Otherwise, we might need to form a "post-atlas trauma support group"!) And, of course, one of the biggest questions we can address will be to look at how bird distribution and numbers change over the coming years-to answer this question, it will be helpful to continue to collect data using the Atlas grid system.

See you at the Elephant Tree Area next winter!

--Lori Hargrove

birdatlas@sdnhm.org

Spring 2002 Wrenderings | Wrenderings Archive | Bird Atlas Introduction