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

WRENDERINGS
The quarterly newsletter for Bird Atlas volunteers
Winter 1999

In this Issue
A Century of Change in the Birds of San Diego County
Modeling the Distribution of the Grasshopper Sparrow—An Application of the San Diego County Bird Atlas

Notable Observations

Reports from the Field
The Story of the Upland Sandpiper
Dove Under Glass
Migrating Swainson's Hawks Take a Rest Stop at Ocotillo Wells
Watching Screech Owls by Day
Willie and Her Commute from Southeastern Oregon to Mission Bay

Focus On...
The Greater and Lesser Yellowlegs

Progress Reports

News and Updates
Bird Art on the Internet
Planning Ahead as the Atlas Passes Its Halfway Point
Prepare to Clear the Winter Threshold in Your Square!
Christmas Bird Counts
Blockbuster Weekends
Wingding Things

A Century of Change in the Birds of San Diego County

The turn of the century inevitably compels us to reflect on the changes in our environment and its birds over the past hundred years. Beyond the arbitrary turning over of time's odometer, the event means something to me personally: it was just over 100 years ago, in 1898, that my ancestors first arrived in San Diego. I grew up with the stories of my great-great-grandmother seeing a Roadrunner trotting down the dirt streets of downtown San Diego, calling it a "streetwalker." The stories of my grandmother swimming across the bay to Tent City, her picking yellow violets and hearing the song of the Western Meadowlark in the canyons now covered by the neighborhood of Golden Hill. The story of my great-grandfather's house on 30th Street in North Park being called "Sheppard's Folly" because it was so far out in the sticks.

The most dramatic change in the San Diego environment over the last century, and the reason for most other changes, is the enormous increase in the human population. In looking for perspective on changes in San Diego County, I found a great source in the U.S. Census Bureau's World Wide Web site. It features figures for both the past and present, plus projections for the future. In 1900 the county's population was 35,000; today it is around 2,800,000, an increase by a factor of 80! Converting this to a percentage increase per year yields 4.6%, unequaled by any third world country. And this is a population consuming water, gasoline, landfills, and living space at least as fast as the rest of Americans.

The southern California environment has been so radically changed by the human population explosion that it's difficult to imagine what birding here could have been like in the 19th century. California Condors still cruised the skies, congregating by the hundreds when 3000 lambs grazing the Warner Ranch were frozen in a snowstorm in 1886. Long-eared Owls still nested commonly in dense riparian woodland, Swainson's Hawks in the more open sycamore groves. The Horned Lark was the commonest bird in Escondido. The White-fronted and Snow Geese wintered abundantly in the valleys of the north county, the Brant on San Diego Bay. A colony of a thousand pairs of Least Terns nested on Mission Beach in 1915; 30 pairs of Black Rails inhabited the estuary of the Sweetwater River on San Diego Bay in 1908. Ten miles offshore, Short-tailed Albatrosses could be seen any day of the year, and sometimes they entered the bay. And these examples are just a selection of the most glaring changes.

Yet the balance sheet has many entries on the plus side as well. The Great and Snowy Egrets, almost exterminated by plume hunters by 1900, have recovered spectacularly. Indeed, many water birds have proliferated, taking advantage of ponds and reservoirs that now dot the courses of once intermittent streams. Species that have increased over the last century are as diverse as the Red-shouldered Hawk, Caspian Tern, Common Ground Dove, Western Flycatcher, and Great-tailed Grackle. Species preadapted to the urban environment, such as Anna's Hummingbird, American Robin, and Brewer's Blackbird, have multiplied greatly. Of course, the arrival or increase of some species isn't necessarily good news. The 1920s saw the population explosions of the House Sparrow and Brown-headed Cowbird, the 1960s that of the European Starling, the 1990s that of the American Crow.

What does the future hold? If the human population of the county continues to grow at the rate of the 1990s (1.35% per year, much slower than the century's average), by 2100 it will reach 10.7 million, which will presumably entail the entire western half of the county being urbanized, much like Los Angeles County, whose current population is 9.25 million. But there are signs that the future may not be quite so bleak as this. Surprisingly, the Census Bureau's figures show that in the past decade migration in and out of the county has virtually equalized, so that births now account for the population's increase. And birth rates are falling worldwide.

Though we can expect many species to decline with further loss of habitat, some will surprise us with their capability to adapt. Just in the last 10 to 15 years, Nuttall's Woodpecker and Cooper's Hawk have joined the list of adapter species. Our atlas results suggest that some others, like Say's Phoebe, could be on the verge of doing so, too. I suspect that most species that make their living in trees will adapt to urbanization to some extent, while those living in undergrowth and on the ground will continue to decline.

Perhaps the most obvious difference of all that strikes me in trying to compare conditions in 2000 to those in 1900 is that we simply don't know--and can never find out--much detail about our environment a century ago. The answers to many questions we would like to ask will remain a mystery. Overcoming this gap is the most fundamental reason for pursuing a bird atlas: leaving a legacy so future generations can evaluate and act on the changes in their environment better than we have been able to do in ours.

 -- Philip Unitt


Modeling the Distribution of the Grasshopper Sparrow—An Application of the San Diego County Bird Atlas

As the species most closely linked to one of the county's most threatened habitats, native grassland, the Grasshopper Sparrow has long been acknowledged as a locally declining species. Recognizing this, Tom Oberbauer of the San Diego County Department of Land Use engaged RECON, a local environmental consulting firm, to develop a hypothetical model for the Grasshopper Sparrow's distribution in the north-central part of the county. Tom asked me to review the report, using atlas data as an independent test of it.

The report proposed that vegetation type is the only environmental variable governing the occurrence of the Grasshopper Sparrow. It ranked grassland, whether it contains native bunch grasses or nothing but foreign invaders like red brome, ripgut grass, or wild oats, as having a high value to the sparrow. It ranked coastal sage scrub and vegetation intermediate between sage scrub and chaparral as low value--sometimes these habitats are broken by grassy openings inhabited by the sparrow. It ranked all other habitats of any significant extent as of no value.

The first obvious result of the comparison is that most of the area addressed in the report, the region from Camp Pendleton and Vista east to the boundary of the Cleveland National Forest, from the Riverside County line south to the northern city limits of Escondido, has little prime Grasshopper Sparrow habitat. Many sites home to the species, from both our atlas database and the database used in generating the habitat maps provided to us by Ogden Environmental, lie just outside of the planning area's boundaries. This itself is a notable result--Grasshopper Sparrow sites are concentrated primarily in the inland valleys from San Marcos and Escondido south to Miramar and Santee, as well as east of metropolitan San Diego from Jamacha to Otay Mesa. The main population of the Grasshopper Sparrow in San Diego County is along the urban growth front and not in more rural areas less subject to imminent development. The report tabulates the distribution of Grasshopper Sparrow habitat within its study area by land ownership--less than 5% of what it identified as high value to the Grasshopper Sparrow is publicly owned.

So, how did prediction and observation measure up against each other? The verdict is mixed. The report identified six areas within its limits with concentrations of possible Grasshopper Sparrow habitat. I believe that three of these correspond to actual Grasshopper Sparrow populations while three do not. Two of the three areas supporting the sparrow are now well corroborated through our atlas effort. Kirsten Winter discovered a major site for it in square A5, where she counted a maximum of 20 on 25 May and 10 June 1999 in only a fraction of the habitat. This is the site at which the model performed best. The Ramona grasslands, not surprisingly, were also identified as likely Grasshopper Sparrow habitat. The model predicted the species in squares K13, K14, L14, and L13, and thanks to the efforts of Phoenix von Hendy and Geoff Rogers the species has been recorded in the first three of these. But it seems likely that the definition of good Grasshopper Sparrow habitat for the Ramona region is too restrictive. The largest concentrations recorded around Ramona in our atlas database are from outside the habitat identified by the model as prime. Margaret McIntosh counted 23 in an area (since developed, sadly) mapped as no probability on 25 May 1998, while Andy Mauro counted 10 in an area mapped as low probability on 10 June 1999. Probably the discrepancies arose from inaccuracies in the vegetation mapping or from open sage scrub in fact good for the sparrow being misrated as low probability.

The model also identified Rancho Guejito (H13, H14, I13, I14) as a major block of likely Grasshopper Sparrow habitat. The ranch is private property to which the public has no access, and we have been unable to cover it for the atlas. So its importance to the Grasshopper Sparrow (and other wildlife) remains conjectural though, I believe from one brief trip across this property, probable.

The other three apparent concentrations of habitat identified in the report, however, I suspect are illusory. The grassland habitat around Pala Vista (F10, F11) and Valley Center (G11, G12, H12) is largely or entirely nonnative and at least in F11 actually disused or intermittently used fields of grain crops. Grassland retaining a significant component of native bunch grasses is far better for the sparrow than secondary grassland (especially that once plowed) vegetated wholly with exotics. Finally, the patch of possible habitat above Pauma Valley on the steep south-facing slopes (not typical Grasshopper Sparrow habitat) of Palomar in D12, E12, D13, and E13 may be just burned scrub.

We have only four additional atlas locations within the plan area's boundary, though there are several others just beyond it. Two of these are a short distance northeast of San Dieguito Reservoir, where Lee Taylor found them in K9, one is near the south boundary of Hellhole Canyon County Park (G13), where Rich Breisch and Lori Hargrove found the sparrows in cleared chaparral regrowing in early successional vegetation, and the last is just north of the Pala Indian Reservation in C11, located by John and Beverly Hargrove. The model identified hardly any high-probability habitat in these areas, implying it did not perform well there.

The analysis was an interesting exercise identifying the possibilities and limits of predicting a species' distribution. Clearly, a more sophisticated model, taking into account the amount of native bunch grass, the percentage of shrub cover, the size of a patch of grassland, and the orientation and steepness of slopes would have greater predictive power. But the way birds look at their habitat is not necessarily the way any human being has looked at it. For example, if a botanist has a choice between classifying an area as grassland or sage scrub, how many shrubs can the area have before he calls it sage scrub rather than grassland? Whatever threshold the botanist chooses, a Grasshopper Sparrow, in deciding what habitat is suitable for it, is probably going to choose something else. The bird's threshold is likely not fixed itself. The species may be restricted to more open habitats in some years and take advantage of shrubbier, less attractive ones in others, when the population has been increased by a high reproductive rate in the previous year--or when the birds have been displaced by nearby development.

In any case, however we seek to understand birds and how they are using their environment, the San Diego County Bird Atlas stands as a body of real empirical data against which such attempts at interpretation can be measured.

 -- Philip Unitt

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