Anna's hummingbirds have been overwintering at higher latitudes only for the last few decades. Prior to the 1930s, it nested no farther north than San Francisco Bay and was not reported north of the Oregon border until 1944. The bird reached Seattle in 1964 and today breeds on Vancouver Island and is found in southeastern Alaska regularly.

Their journey north appears to have begun with the appearance, and then northward establishment, of another species: the blue gum eucalyptus tree from Australia. First introduced to southern California in the 1870s for shade, lumber, and railroad ties, and later used for lumber and orange-grove windbreaks, the tree is now naturalized in the coastal areas of southern California and the San Francisco Bay region. Areas of the state that were once treeless plains are now savannahs or long-abandoned plantations of blue gum.

The tree’s nectar-rich flowers bloom in the winter. Anna’s Hummingbird is one of only two native wildlife species that appear to find value in the tree. The Monarch butterfly, which uses it as a winter roost, is the other. Taking advantage of a developing urban horticulture in the Los Angeles Basin, Anna’s found it could now live year-round in the lowlands of southern California and later move north to the Bay area as blue gum groves there matured.

In their original southern California habitat, Anna’s Hummingbirds rely on chaparral and gooseberry, both with long growing seasons, but they readily forage on exotics provided by local nurseries. The birds have simply shifted north to capitalize on the profusion of urban gardens in the Pacific Northwest. The Seattle area’s locally large populations of eastern gray squirrels and American Crows have similarly adapted to the regional shift from conifer forest to the combined native and exotic deciduous trees.

https://www.birdwatchingdaily.com/news/species-profiles/annas-hummingbird-our-winter-hummingbird/