Southward and downslope extensions of breeding ranges of birds in southern California

Range changes that appear contrary to predictions based on climate warming deserve examination for any shared factors opposing the general trend. Of the ~273 species of native birds currently or historically nesting regularly in southern California, there is, in the historic record, some level of evidence for a southward or downslope range extension in 52. A few apparent extensions may be illusory, based on more thorough exploration since the mid-20th century, but the great majority represent actual changes. These range from minor, involving only a few pairs at a few localized sites, to extensive, as in the cases of the Western Grebe (Aechmophorus occidentalis), Pacific-slope Flycatcher (Empidonax difficilis), American Crow (Corvus brachyrhynchos), Western Bluebird (Sialia mexicana), American Robin (Turdus migratorius), and Orange-crowned Warbler (Oreothlypis celata). Some expansions began as early as the 1930s (American Robin, Orange-crowned Warbler), whereas others, such as that of the Canada Goose (Branta canadensis), have gained momentum only in the 21st century. Roughly half of the species with expanding ranges are associated with coniferous forest or other montane habitats. Some of these expansions may be associated with the increasing density of California’s forests where not burned or logged, but others are by montane species that use shrubby habitats or open forest. Eighteen of the species with expanding ranges are associated with water, capitalizing on the many reservoirs, sewage ponds, and other water bodies created for importation, management, and control of water in southern California. Ten species exploit the urban habitat that now pervades southern California, but most of these have spread in natural or lightly modified habitat as well. We conclude that habitat modifications have been a leading force driving southward and downslope range expansions of birds in southern California.