Urban Adaptation of the Western Bluebird

By Carol Killebrew, San Diego County Coordinator for the California Bluebird Recovery Program

According to the North American Breeding Bird Survey, Western Bluebirds are numerous and populations have been increasing 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.

Range: Southern British Columbia through much of western U.S. into Mexico. (map by Paul Lehman)

Over much of its range the Western Bluebird population trend appears to be increasing. This is a reversal of the earlier decline as a result of loss of nest cavities to logging, fire suppression, and from competition for cavities from nonnative 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, despite many competitors for nest sites, the Western Bluebird appears to be extending its breeding range. 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.

Western Bluebirds are fascinating, lovely songbirds. To attract them you will need to put up a nest box specifically designed for Western Bluebirds in the appropriate location and monitor it. You may also need to supply water and protect the birds from pests. You’ll find information about how to get started with Western Bluebirds (and more) at the “Bluebirds of San Diego County” website.