Short-distance Migration
Hooded Oriole ©Ann Brice
Fall migration, the most exciting season for birding in these parts, is in full swing. Birds from far away visit Yolo County for a short time, on their way from summer to winter quarters. And there are three other kinds of migration, accomplished by species normally considered non-migratory.
The first is a short-distance commute between breeding and wintering grounds. An example is Hutton’s vireo, which is an occasional winter visitor where I live in Davis, that nests in the nearby foothills. Similarly, spotted towhee is in my patch only in winter, as the nearest breeders are some miles away. A pair of red-shouldered hawks nested on my block this past spring. After the young fledged, the entire family disappeared to who knows where, and I haven’t seen them since.
A second kind of migration of normally sedentary birds is a gradual shift in geographic range. In recent decades this has usually meant expansion northward. Before roughly the 1960’s, southern species like mockingbird, Anna’s hummingbird and red-shouldered hawk ranged no further north than the head of the Sacramento Valley. The end of the range of black phoebe was Trinity County, while that of acorn woodpecker was southwestern Oregon, and wrentit got just a bit further.
Now the hawk and the mockingbird are somewhat common in western Oregon and can be found occasionally in western Washington. The hummer is common all the way to Vancouver BC. Black phoebe occurs regularly into western Washington. The woodpecker is resident in much of Oregon and the southern edge of Washington. Wrentit has moved further up to the north edge of Oregon. The case of the red-shouldered hawk reminds us that species can perform multiple kinds of migration.
Some species have reached Yolo County as part of the northward push. Eighty years ago, white-tailed kite got no further than Santa Clara County and hooded oriole stopped near Modesto. Now the kite occurs widely in northern California and coastal Oregon and the oriole breeds throughout the Sacramento Valley and much of coastal northern California.
Other than hooded oriole, all of these species are year-round residents wherever they live. As warm-climate birds, the colder edge of their range is probably set by the severity of winter. This suggests that northern winters have gotten milder in recent decades, from their perspective. The exception, the summer-visiting oriole, may not be a true exception. Its favorite nest tree, the fan palm, has been planted everywhere that the palm can tolerate winter cold, its distribution almost exactly matching the summer range of the oriole.
Finally, sedentary species sometimes migrate short distances within their overall range. White-breasted nuthatches arrived in my neighborhood only recently, shortly after the last big wildfire in the western hills. They appeared that fall and stayed to nest. Most likely the fire scattered them, and many other birds as well. Western bluebird is another fairly recent arrival in my local patch. This doubtless owes to the nest boxes put up here a few years ago. Similarly, only after nest boxes appeared at the Putah Creek Riparian Reserve at UC Davis were bluebirds found in spring and summer.
Bird distribution is seldom static on any geographic scale. Follow the birds on a patch or two, year after year, and see for yourself.
–Michael Perrone, Conservation Chair