From 1912 to 1923, Joseph Grinnell and fellow ornithologists at UC Berkeley surveyed birdlife in spring and summer at forty-one stations in the Central Valley, producing a baseline list of the breeding birds of the region. One hundred years later, another UC Berkeley group revisited the same stations at the same seasons. A member of that group, Sarah MacLean, has compared the two surveys in detail. Because nearly all of the wetlands and grasslands of the Valley have given way to farms and cities, she expected to see large differences between the two bird lists.
And so it turned out. Birds suited to human-associated landscapes grew much more common, occupying many more of the survey sites. These “human adapters” include Canada goose, great egret, red-shouldered hawk, Anna’s hummingbird, mourning dove, cliff swallow, raven, crow, mockingbird, brown-headed cowbird and great-tailed grackle, as well as the introduced ring-necked pheasant, wild turkey, collared dove, rock pigeon, and starling. It may come as a surprise that these familiar Yolo County birds were scarce a century ago.
What was lost in the trade was an astonishing number of birds, including species of open country (turkey vulture, American kestrel, burrowing owl, roadrunner, Say’s phoebe, loggerhead shrike, western meadowlark, lark sparrow), species of marshes (yellowthroat and yellow-headed blackbird), woodland species (flicker, Lewis’s woodpecker, hairy woodpecker, purple martin, blue-gray gnatcatcher, dark-eyed junco, chipping sparrow, American goldfinch), and species of thickets (Bell’s vireo, California thrasher, yellow-breasted chat, lazuli bunting).
Moreover, the study undercounted the species that have been lost, because it misclassified the current or former status of several birds. The analysis reported willow flycatcher and yellow warbler as current breeders, rather than as the migrants-only that they are today. Swainson’s thrush was omitted from analysis for being strictly a migrant, but should have been included as a former breeder.
It is remarkable (and seldom recognized) that so many species were common or fairly common a century ago, but scarce or absent as breeders today. The causes are varied. Purple martins probably lost nest sites to starlings. Habitat loss was important, particularly for open-country, thicket and marsh birds. A compounding factor is that many small songbirds were decimated by cowbird parasitism of their nests and, perhaps less often, nest predation by jays. Cowbirds normally parasitize smaller birds, so it is no coincidence that all but two of the human adapters are bigger than cowbirds. The two, Anna’s hummingbird and cliff swallow, are not parasitized by cowbirds.
The study author believes that today’s bird diversity in the Valley is propped up by the presence of artificial water features (canals and reservoirs) that substitute for wetlands, and by the well-watered landscaping in cities and towns.
Note that the analysis focused on land birds, because the survey method was good at detecting them. It omitted shorebirds, divers, and most waterfowl. Theirs is a story for another day.
The study, “A century of change in avifauna of California’s most transformed landscapes”, can be accessed here.
Michael Perrone, YAS Conservation Chair