Bluebirds of San Diego County

Western Bluebird (Sialia mexicana)

Male Western Bluebird by CM KillebrewMale Western Bluebird by CM Killebrew

Breeding Distribution

Western Bluebirds can be found breeding from the Rocky Mountains to the West Coast. Their extremely large 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 population trend appears to be stable or increasing. Declines appear to be a result of loss of habitat to logging, fire suppression, and from competition for cavities from non-native European Starlings and House Sparrows. 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.

Female Western Bluebird by L. NightwyndFemale Western Bluebird by W. Esterly

Urban Adaptation in San Diego County

Phil Unitt, curator of birds at the San Diego Natural History Museum, has 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.

Ornithologists are also observing what may be a general southward range expansion for the Western Bluebird, paralleling that of the Orange-crowned Warbler and Pacific-slope Flycatcher. In 1919 Western Bluebirds were only common in the mountains of San Diego County during the winter, but now they are common in the inland valleys, foothills, and mountains year-round.

juveniles and adult male by Z. Poulsonfemale and male juveniles with adult male by Z. Poulson

Range: Southern British Columbia through much of western U.S. into Mexico.

Red/Summer; Blue/Winter; Purple/Year Round. Map by Paul Lehman.

regional map

According to the North American Breeding Bird Survey, Western Bluebirds are numerous and populations have been stable since 1966. Partners in Flight estimates a global population of 6.7 million with 67 percent spending some part of the year in the U.S., 52 percent in Mexico, and 1 percent in Canada.

Threats to the Western Bluebird primarily stem from the loss of habitat both from extensive logging and from growth of forests as a result of the suppression of natural fires; also development and grazing have reduced habitat availability. Even in appropriately wooded habitat, people may remove valuable dead trees in an effort to clean up; this limits the places where bluebirds and other cavity nesters can find nest sites. Aggressive, non-native cavity nesters such as House Sparrows and European Starlings may take over many of the nest sites that Western Bluebirds might otherwise use.

Audubon recently designated the Western Bluebird (along with 314 other North American bird species) as climate threatened, as it could lose some of its range by 2080 due to climate change. Researchers believe that the birds can adapt to new areas, but it will be important for us to preserve forest and tree health to ensure that habitat is available.

Female Western Bluebird by S. BradMale Western Bluebird by S. BradJuvenile Western Bluebird by S. Brad

Taxonomy

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.