Western Bluebirds can be found breeding from the Rocky Mountains to the West Coast. Their range extends from southern British Columbia to central Mexico. They are most abundant along the California coast and in the southwestern states. Except during breeding season, the Western Bluebird often travels in small flocks, feeding on insects and berries. Over much of its range the Western Bluebird has been in decline, as a result of loss of nest cavities to logging, fire suppression, and from competition for cavities from non-native European Starlings and House Sparrows. However the Western Bluebird is not considered a "threatened" or "endangered" species. Its current conservation status is listed as "Least Concern" by the IUCN Red List. And in San Diego County, California, despite many competitors for nest sites, the Western Bluebird appears to be extending its breeding range.
Phil Unitt, curator of birds and mammals at the San Diego Natural History Museum, recently published a nice book: San Diego County Bird Atlas. In his book, Unitt states that summer and winter bird counts since the late 1990's indicate the Western Bluebird is holding its own in the foothills and mountains of San Diego County, and showing signs of spreading into urban areas with mature trees and wide lawns. He says in the late 1980's, Nuttall's Woodpecker started adapting in San Diego County, moving into the city wherever it was landscaped with wood-pecker friendly trees like liquid-amber, birch, alder, eucalyptus and even agave. This cavity excavator helped pave the way for two secondary cavity nesters, the House Wren and Western Bluebird.
Unitt also says that more people are putting up birdhouses, making the nesting boxes an increasing factor in the spread of the House Wren and Western Bluebird. He also sees a pattern emerging: many arboreal species that can live in a stratum above us people on the ground ultimately adapt to urbanization, while terrestrial and undergrowth species retreat.
Bluebirds are native only to North America and belong to the large order of Passerines, often referred to as "songbirds" or "perching birds." Western Bluebirds, along with Mountain Bluebirds and Eastern Bluebirds, belong to the genus Sialia which is part of the Turdidae (thrush) family. Bluebirds are closely related to Robins, which are also members of the family Turdidae. [Note: the Mountain Bluebird is an irregular visitor to our grasslands in the winter, but does not breed in San Diego County.]
The scientific name for the Western Bluebird is Sialia mexicana. The Western Bluebirds in San Diego County, California are of the Pacific Coast subspecies Sialia mexicana occidentalis (Townsend 1837), which has the reduced chestnut on the back. The Rocky Mountain Western Bluebird, S. m. bairdi, with extensive chestnut on the back (Ridgway 1894), may occasionally mix into S.m. occidentalis winter flocks in Joshua Tree National Park, but does not breed in California.
Male Western Bluebirds wear plumage of deep ultramarine blue on the head, back, wings, tail, and throat. In some males, blue extends down the center of the breast to the upper belly. Chestnut orange feathers decorate the breast and the flanks and, in some individuals, form a chestnut patch above the shoulder. The belly and undertail are grayish white. Female Western Bluebirds have gray-brown plumage on the head, back, and throat. Their wing and tail feathers are azure blue, shaded with gray. Breast feathers are chestnut orange. Juveniles are spotted in typical thrush pattern, molting into adult plumage in the fall.