from the Field
The Winter PhaseA Unique Aspect of the San Diego Bird Atlas
As you may be aware, atlases of breeding birds have been completed or are being pursued in many areas not only of California and the rest of the U.S. but in some other countries, too. an effort to record the distribution of wintering birds in the same system, however, is nearly unique to San Diego County. Certainly no other such project is underway elsewhere in California, and the only parallel I know of elsewhere in North America is from the province of Québec, where observations over several years were recorded in a grid system. but apparently the Québecois made no effort to inventory each square to a defined minimum threshold, as we are attempting in San Diego. One must cross to the opposite side of the world, to France and Britain, to find examples close to ours.
So we must ask the question, why pursue a winter bird atlas? Most species have more exacting requirements and more precisely defined distributions during their breeding season. Nevertheless, winter habitat is as essential to birds as breeding habitat. It's quite possible for the numbers of some species to be as limited by winter habitat as by breeding habitatfor a hint of this, one need look only at a map for the many species of North American birds that pack into a winter range in souther Mexico or Central America a fifth or a tenth the area of the breeding range.
San Diego County has more species of wintering birds than breeding birds. Might not this area be as critical to some wintering species as the dwindling tropical forests of Central America? Read the histories of the vast numbers of geese wintering at San Diego in the 19th century. glance in winter at our remaining coastal wetlands, still often packed with ducks and shorebirds. Squeak in mountain chaparral and watch the fox sparrows and Hermit Thrushes emerge from under nearly every shrub. Such concentrations suggest the answer may be yes. The Fox Sparrow is a perfect examplebirds breeding all over western North America pack in for the winter to California's chaparral.
Winter ranges are known more poorly than breeding distributions. For example, in my book The Birds of San Diego County published in 1984, I mapped breeding locations as far as then known but not winter ranges. In neither Garrett and Dunn's 1981 Birds of Southern California or Grinnel and Miller's 1944 Distribution of the Birds of California are there maps of winter ranges either. What few maps have been published, mainly in field guides, are at a very gross scale.
The line between sedentary and migratory species is not sharp. Many species that breed in San Diego County use their habitat differently in winter than in summer. Many are augmented by winter visitors arriving from farther north. The entire breeding population may be replaced by winter visitors. The Virginia Rail, Phainopepla, Western Bluebird, Orange-crowned Warbler, Lark Sparrow... just a few examples of the many species whose complex distributions will be far better illuminated with our winter atlas data.
Winter habitat can be critical for conservation, too. The Mountain Plover, possibly meeting the criteria of an endangered species, wintered regularly in San Diego County until just a few years abo. Are there any sites for it left? Adequate habitat for foraging raptors has become an issue. Where is there still sufficient habitat to support those that need wide open spaces like the Ferruginous Hawk and Prairie Falcon? Are Canada Geese still trying to use their traditional sites, even as these are consumed by urban sprawl, or are they shifting successfully to new sites and new habitats? The winter phase of the San Diego Bird Atlas should give much clearer answers to all these questions than are currently possible.
Finally, our winter results will provide the data with which some broader scientific questions can be addressed, questions relevant to successful conservation. In the special October 1998 California Gnatcatcher issue of Western Birds, Pat Mock raises the issue of whether it is the habitat or the climate limiting the gnatcatcher's distribution. Several studies published in this issue reveal that the largest source of mortality of adult California Gnatcatchers is cold, wet winter weatherapparently, the birds just die of exposure. Pat found that the gnatcatchers are rare or absent at sites where the average January low temperature is 2.5°C. or less, even where sage scrub looks suitable for them. Clearly, successful conservation of a species cannot be based on habitat where it is likely to die of hypothermia. Do such climatic limitations apply to many other species? San Diego county, with its enormous variations in climate, is a natural laboratory for studying this question,and our winter atlas data are the raw material for the experiment.