Sialia mexicana
Year-round resident
Western bluebirds are vibrant residents of Edgewood’s grasslands and woodland edges, often seen kiting over the grass or on low, exposed perches scanning for insects.
Appearance
Western bluebirds are characterized by their blue plumage, rusty-orange breasts, and white stomachs, though the overall brightness varies significantly depending on age and sex. This species exhibits sexual dimorphism—a term used when males and females of the same species display different physical characteristics (color, size, etc.).
Adult males are generally the most striking, sporting deep cobalt blue with a rich chestnut-red breast that sometimes wraps around the upper back. In contrast, adult females are generally more subtle, with a lighter blue body and a muted orange breast. However, identifying them isn’t always clean-cut; because of individual health, age, or even how the light hits their feathers, you will often see a lot of intermediate coloring and overlap between the sexes.
Juveniles are the most distinct, featuring spotted or scaly grayish-brown feathers that lack the solid blocks of color found on adults, though they still show telltale hints of blue in their flight feathers.
ID Tips
You might confuse a Western bluebird with a California scrub-jay (Aphelocoma californica) or a Steller’s jay (Cyanocitta stelleri), other blue-colored birds at Edgewood. California scrub-jays are much larger and louder, plus they have a gray back patch and a white breast. Steller’s jays are also much larger and louder, but they are distinguished by their black, crested heads and deep blue chests; they also tend to hang out much deeper in the woodlands than the sun-loving bluebirds.
At Edgewood
Western bluebirds thrive at the edges of Edgewood’s woodlands. They prefer habitats where there are scattered trees for nesting but plenty of open ground for hunting, and they often use the many nest boxes found throughout the preserve. See iNaturalist and eBird for observations of this bird at Edgewood Park and Natural Preserve.
Diet
Bluebirds need “lookout” perches—like a low oak limb, a fence post, or a sign—where they can sit and scan the ground for movement. They are drop-foragers, meaning they sit patiently on a perch until they spot an insect, then flutter down to the ground to snatch it. You’ll also see them kiting—hovering in one spot over the grassland before diving to catch a tasty morsel.
Their diet consists almost entirely of insects like grasshoppers, beetles, and caterpillars during the summer. In the winter, when insects are scarce, they pivot to eating berries. In the winter at Edgewood, look for them on berry-producing plants like blue elderberry (Sambucus mexicana), toyon (Heteromeles arbutifolia), and poison oak (Toxicodendron diversilobum).
Behavior
Western bluebirds are socially monogamous and often form long-term pair bonds. However, their social loyalty doesn’t always mean genetic loyalty; experiments reveal that up to 45% of nests contain one or more young that are not the resident male’s offspring (Dickinson 1998). To protect his lineage, the male will stay close and follow the female during her fertile period—a behavior called mate-guarding—especially when other males are nearby (Dickinson 1996). While both parents provision the nestlings, only the female broods the young.
Their social structure can occasionally include cooperative breeding; a California study showed that about 7.4% of pairs had helpers. These helpers are usually young males from a previous year’s brood (Dickinson 1996). This is a flexible strategy used when conditions are tough or territories are scarce, rather than a standard social requirement.
Despite their gentle appearance, Western bluebirds can be fierce defenders of their space. Males may grab opponents’ legs in mid-air, causing both to fall to the ground, where the aggressor may pin his opponent down and strike vigorously with his bill (Dickinson 1996).
Conservation
The vibrant blue of a Western bluebird is a common sight at Edgewood today, but it wasn’t always so. In the late 20th century, populations across California were declining due to a lack of nesting cavities—a consequence of habitat loss.
Unlike many birds that weave nests from twigs and grass in the open air, Western bluebirds are secondary cavity nesters. Lacking the beaks needed to excavate wood, they act as the ultimate tenants—relying entirely on pre-built housing like abandoned woodpecker cavities, natural rot-hollows, or nest boxes.
To combat this housing crisis, the California Bluebird Recovery Program (CBRP) was launched in 1996. Following this initiative, Lee Franks of the San Mateo County Bird Alliance established Edgewood’s first bluebird trail in 1997 with strategically placed nest boxes designed to mimic natural cavities. The project was a resounding success, and the birds soon returned to the preserve.
Today, the Friends of Edgewood continue this legacy through a dedicated monitoring program. You may spot these nest boxes mounted on poles, fences, or trees throughout the preserve’s grasslands and woodland edges. Each year, our volunteers meticulously track nesting success, and you can check out the annual results of this monitoring every December in our newsletter, The Edgewood Explorer.
Fun Fact
Unlike many other bird colors, the vibrant blue of a Western bluebird is not caused by a blue pigment. It’s a structural color caused by the way the feather is built. Microscopic structures in the feather scatter light in such a way that only the blue wavelengths are reflected back to your eyes. If you find a blue feather on the ground, try this experiment: shine a light from behind the feather. You will see that the “blue” disappears and the feather looks gray or brown! Learn more about how birds make colorful feathers.
Learn More
Learn more about Western bluebirds: All About Birds and Western Bluebird Photo Gallery.
Learn more from these organizations: California Bluebird Recovery Program, North American Bluebird Society, and San Mateo County Bird Alliance.
Learn more about Edgewood’s bird counts: Edgewood Explorer, December editions.
References
Cornell Lab of Ornithology. 2015. How birds make colorful feathers. Bird Academy.
Dickinson, J.L., and J.J. Akre. 1998. Extrapair paternity, inclusive fitness, and within-group benefits of helping in Western bluebirds. Molecular Ecology 7: 95-105.
Dickinson, J.L., W.D. Koenig, and F.A. Pitelka. 1996. Fitness consequences of helping behavior in the Western bluebird. Behavioral Ecology 7: 168-177.
Dickinson, J.L., and M.L. Leonard. 1996. Mate attendance and copulatory behavior in Western bluebirds: Evidence of mate guarding. Animal Behaviour 52: 981-992.
Guinan, J.A., P.A. Gowaty, and E.K. Eltzroth. 2020. Western Bluebird (Sialia mexicana), version 1.0. A.F. Poole (Ed.). Birds of the World. Cornell Lab of Ornithology, Ithaca, New York.