Looking over decades of field notes from my neighborhood in Davis recently, I was reminded of two changes in bird life in this period in the Sacramento Valley. The first is the growing numbers of over-wintering birds here that are usually found in the milder climates of the central California coast and further south. The second is that our winters have gotten warmer. In particular, multi-day periods of tule fog have disappeared, giving way to milder, sunny weather.
Twenty years ago I could scarcely find turkey vulture, white-throated swift, Cassin’s vireo, blue-gray gnatcatcher, black-throated gray warbler, Townsend’s warbler or western tanager in winter, and house wren and orange-crowned warbler were few and far between on my walks. Today none of these birds is rated rare in winter in eBird. In other parts of the county, add white-faced ibis, barn swallow, rough-winged swallow and yellow-headed blackbird to the list of species that nowadays linger late into the fall or stay through the winter. In line with that trend, in December and January birders enjoyed the first Yolo County winter record of a distinctly southern bird, the summer tanager.
The fifty-two years of the Putah Creek Christmas Bird Count tell the same story as my neighborhood. (Big thanks to Yolo Audubon member Bart Wickel for putting the data in an easy-to-understand pictorial form.) The numbers of house wren and orange-crowned warbler have increased continually since about 2000. The bulk of the western tanager, Townsend’s warbler and black-throated gray warbler records have been in the last twenty years. Starting from very small numbers, turkey vulture began to increase in 1987, first exceeded two hundred birds in 2000, and continues to climb. At about the same time, white-throated swift became quite scarce on the Christmas Count, and has stayed scarce.
Some of the connections between birds and fog and its absence are straightforward. Vultures and swifts previously stayed in the Coast Range hills, avoiding the valley fog that prevented their aerial foraging. When the fog left, the swifts abandoned their cliffy roost sites for highway overpasses in the valley, thus nearly disappearing from the Putah Creek Christmas Count. All the other birds under discussion, except the ibis, need insects in their diet. It seems likely that milder fall and winter weather is a boon for many kinds of bugs and thus for the birds that eat them. Consistent with this diet-based explanation, the two seed-eating species at the north end of their winter range here, chipping sparrow and vesper sparrow, showed no trend in numbers.
One insectivore didn’t entirely follow the rule. Blue-gray gnatcatcher did not begin to stay the winter in my home patch in Davis until 2007, but has not become more numerous on the Christmas Count. Maybe the milder weather has failed to help gnats all that much.
This seasonal and regional shift begs a question. If our winters are now more attractive to mild-weather southern birds, have they become less attractive to cold-hardy northern species? Watch this column for some findings.
Black-throated Gray Warbler
photo by JoAnne Fillatti
— Michael Perrone, Conservation Chair