White-crowned Sparrow, Timber, Washington County, Oregon on 28 June 2008 by Greg Gillson.The handsome White-crowned Sparrow is not overly common at backyard feeders in the Pacific Northwest in winter. But migrant birds pass through in April and September, often visiting backyard feeders in good numbers. West of the Cascades and especially along the coast these are common summer birds.
But this is an over-simplification of a fascinating population dynamic. If you look at the range map of White-crowned Sparrow in North America, you'll see birds breed from Newfoundland and across northern Canada through Alaska. From there they breed in the Rocky Mountains to northern Arizona and New Mexico, and also along the West Coast to southern California. Northern birds winter to the south, to the central latitudes of the interior US, along both US coasts, and into Mexico.
Ornithologists divide the White-crowned Sparrow into 5 populations. Each has a slightly different range, plumage, and song.
It is a bit complicated, but in the Pacific Northwest the birds breeding in the Rocky Mountains and mountains in the Great Basin are the "Mountain White-crowned Sparrow." This form has black lores (feathers between the bill and eye). All other regular forms in the Pacific Northwest have pale lores. They depart for southern regions in winter.
Spring (April) and fall migrants and occasional wintering birds, primarily east of the Cascades, are the "Gambel's" form nesting in Alaska. These winter in the deserts of the American Southwest and into Mexico.
West of the Cascades from southern British Columbia to northwestern California breeds the "Puget Sound" form (as photographed above). Some birds remain to winter, but most migrate south into southern California.
From Mendocino County, California, south along the immediate coast lives the non-migratory "Nuttall's White-crowned Sparrow".
Many of the birds in fall and winter are immatures with rusty brown and white stripes, rather than black and white. These young birds can be confused with immature Golden-crowned Sparrows. Likewise, the rather rare-in-the-West White-throated Sparrow is very similar, but with a blackish bill and white throat clearly set off from a grayish breast.
White-crowned Sparrows eat black oil sunflowers and other seeds at your backyard feeder. This may allow you to examine them more closely, perhaps noticing differences related to age or population as discussed above. For more details see especially the National Geographic Field Guide to the Birds of North America.