Sialia mexicana

Geographic Range

Sialia mexicana is found throughout parts of western North America, including southeastern British Columbia, Alberta, Washington, Oregon, Idaho, California, Nevada, Utah, Colorado, New Mexico, Arizona, northern Baja California, and the central Mexican states.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Western bluebirds are often found at the edges of forests. Because of the availability of perch sites, they tend to live in burned and logged areas. Western bluebirds tend to build their nests in conifer trees (pine and firs) and deciduous trees (oak). They do not live in open areas, such as meadows. They can be found at elevations up to 2,900 m.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest
• scrub forest scrub forest

Physical Description

Western bluebirds are small thrushes ranging in length from 16.5 to 19 cm and weighing from 24 to 31 g. Male and female adult western bluebirds differ in their coloration. The females are less dramatic than the males. They tend to have a brown abdomen and a gray head, throat and back. Their tails and wings are a gray-blue color. The males are brighter. Cobalt-blue is found on their head, chin, throat, and tail. The males have a brown breast and a gray-blue belly. Juveniles exhibit the same patterns as adults, except their coloration is not as intense.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes colored or patterned differently
• male more colorful

Reproduction

Western bluebirds are monogamous; they are also co-operative breeders ("helpers" or birds that are not the parents help to raise young). In some instances extra-pair copulation occurs (matings outside of the monogamous relationship). Helpers reduce the chance of extrapair copulations. They tend to fend off male intruders when the male mate is away from the nest. The females often use display signals to deter extrapair copulations (Dickinson et al., 2000). Females have been seen leaving their perch sites and attacking the breast of an invading male, as well as flattening themselves against a branch. The invading males typically flap their wings rapidly and call in a high-pitched tone (Dickinson et al., 2000). Most attempts at extrapair copulation fail because the female is not receptive. However, in a small percentage of cases extrapair copulations occur. If the intruding male is older than the female, then the female is more likely to mate with him (Dickinson, 2001).

• Mating System
• monogamous monogamous
• cooperative breeder cooperative breeder

Both male and female western bluebirds can begin reproducing once they are a year old. Breeding occurs from May to July. Females typically lay one to two clutches (approximately 5 eggs per clutch (range 3 to 8)) during the breeding season. Copulation occurs from 10 days prior to egg laying through the last day of egg laying (Dickinson et al., 2000).

Incubation lasts from 12 to 18 days (Guinan et al., 2000). The nestlings fledge after 21 days, but they remain close. After two weeks, the young birds are fully independent (Guinan et al., 2000).

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• oviparous oviparous

Female western bluebirds are responsible for incubating the eggs. Incubation lasts from 12 to 18 days (Guinan et al., 2000). During incubation, males guard the nest while the female searches for food. The female does not leave the nest for too long because the males do not incubate the eggs. Once the eggs hatch, both parents are responsible for taking care of the altricial young by cleaning the nest and providing food. Females also brood the nestlings. The chicks are able to leave the nest after 21 days, but they remain close. After two weeks, the young birds are fully independent (Guinan et al., 2000).

In addition to the mother and the father caring for their young, western bluebirds often have helpers at the nest (Kraaijeveld and Dickinson, 2001). These helpers are older offspring, who are non-breeders. The helpers may have been reproductively active at one point in time, but became helpers because their mate died or their nest failed. They increase their fitness by ensuring the survival of the fledglings who are related to them. For example, if a male bird does not reproduce, he is not passing on any of his genes so his fitness is zero. However, if a male bird helps at his parents nest, then some of his genes will be passed on because he is related to the fledglings. A helper increases his fitness by taking care of his siblings (Kraaijeveld and Dickinson, 2001).

• Parental Investment
• no parental involvement
• altricial altricial
• pre-hatching/birth
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • female
• pre-independence
  • provisioning
    • male
    • female

Lifespan/Longevity

We do not have information on lifespan/longevity for this species at this time.

Behavior

Western bluebirds migrate over short distances. From July to October, western bluebirds migrate to their winter nesting sites, and from February to March they return to their spring nesting sites. Birds living at high elevations move to lower elevations in the winter in search of food. During the winter, western bluebirds live in kin groups (small groups of related birds). These kin groups provide protection from predators. The size of the group depends on the availability of resources. In addition to providing protection, male western bluebirds use the kin groups to find a mate or a pair of birds to help at their nest. Immediately after the winter, western bluebirds migrate to higher elevations to mate or to help out at another nest. Western bluebirds are territorial during the breeding season.

In addition to the mother and the father caring for their young, western bluebirds often have helpers at the nest (Kraaijeveld and Dickinson, 2001). These helpers are older offspring, who are non-breeders. The helpers may have been reproductively active at one point in time, but became helpers because their mate died or their nest failed. They increase their fitness by ensuring the survival of the fledglings who are related to them. For example, if a male bird does not reproduce, he is not passing on any of his genes so his fitness is zero. However, if a male bird helps at his parents nest, then some of his genes will be passed on because he is related to the fledglings. A helper increases his fitness by taking care of his siblings (Kraaijeveld and Dickinson, 2001).

• Key Behaviors
• flies
• motile motile
• migratory migratory
• territorial territorial
• social social

Home Range

We do not have information on home range for this species at this time.

Communication and Perception

Western bluebirds use a variety of calls to communicate. Communication begins as early as 14 days as nestlings call for food. Calls become more complex as western bluebirds grow older. There are a variety of calls that are used to determine the location of mates as well as to establish territorial boundaries. The "Kew" call and the "Che-check" call are used by mates to determine each other’s location. These calls are particularly important during breeding season. Additional calls are used to establish territorial boundaries. When building a nest, males produce a chatter call to establish their territories. If a foreign male invades another birds’ territory, the defending male will produce a squawk-like call. Calls are the common method of communication among western bluebirds and are used to establish boundaries as well as position.

• Communication Channels
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Western bluebirds eat a variety of foods; their main food source varies depending on the season. During the spring and summer months, they are insectivorous. They are "perch-foragers" at this time; they will look for prey from a perch and then drop down on the ground to pick it up. During the winter, western bluebirds mainly eat fruit. Their main source of food in the winter is mistletoe berries ( Phoradendron ).

Foods eaten include: spiders, flies, grasshoppers, bees, sowbugs, beetles, termites, mistletoe berries, cherries, blackberries, raspberries and figs.

• Primary Diet
• omnivore omnivore

• Animal Foods
• insects
• terrestrial non-insect arthropods

• Plant Foods
• fruit

Predation

Chipmunks (grey-necked chipmunk ( Eutamias cinereicollis ), townsend chipmunk ( Eutamias townsendii ) and yellow-pine chipmunk ( Eutamias amoenus )), squirrels (Douglas squirrel ( Tamiasciurus douglasi ), red squirrel ( Tamiasciurus hudsonicus ) and tufted-eared squirrel ( Sciurus aberti )) and mice (deer mouse ( Peromyscus maniculatus )) are predators of western bluebirds. They will attack chicks in a nest and then take over and live in the nest. Male western bluebirds stand guard at the nests and chase intruders away. While an intruder is running away, the male will grasp the intruder's legs causing it to fall. Then, the bird will attack the predator with his beak.

Ecosystem Roles

Western bluebirds live in areas that are occupied by other birds. Conflicts arise between swallows, wrens, woodpeckers, and flycatchers for nest space and food. Swallows and wrens have taken over some western bluebird nest sites because space is limited. Western bluebirds and mountain bluebirds have overlapping ranges, but western bluebirds establish their nests earlier, so they usually win disputes over nesting sites.

Economic Importance for Humans: Positive

Bluebirds are beautiful birds and people often put out nest boxes to attract them to their area.

Economic Importance for Humans: Negative

There are no known adverse affects of western bluebirds on humans.

Conservation Status

Western bluebirds are not endangered; however, their habitat is threatened. Western bluebirds do not live in open areas; they live in the forest (Guinan et al., 2000). However, increases in logging have led to increases in the amount of open area in the forest, which has led to a decrease in the number of available nesting sites for western bluebirds. In addition to the loss of nesting sites, the suppression of forest fires has led to a decrease in edge habitat (Guinan et al., 2000). To prevent further declines in western bluebird populations, nest boxes have been set up throughout California to provide breeding sites (Dickinson, 2001). Western bluebirds are protected under the US Migratory Bird Treaty Act.

Other Comments

The genus Sialia has three species: western bluebirds ( Sialia mexicana ), mountain bluebirds ( S. currucoides ), and eastern bluebirds ( S. sialis ). These three species differ in their coloration and body shape. Males of the different species are easily recognizable; females are more difficult to distinguish. For example, male eastern bluebirds have a red-brown throat and a white belly instead of a blue throat and grayish-blue belly. Female eastern bluebirds have an orange-brown throat and a white belly with a pale brown outline. Mountain bluebirds are longer and have a thinner beak. The males and the females have a paler blue coloration. Body size and color patterns are important features in distinguishing between the different bluebird species.

Additional Links

Encyclopedia of Life

Contributors

Alaine Camfield (editor), Animal Diversity Web.

Tiffany Musick (author), Western Maryland College, Randall L. Morrison (editor), Western Maryland College.

References

Bildfell, R., E. Eltzroth, J. Songer. 2001. Enteritis as a cause of mortality in the western bluebird (Sialia mexicana). Avian Diseases , 45: 760-763.

Dickinson, J., K. Kraaijeveld, F. Smit-Kraaijeveld. 2000. Specialized extrapair mating display in western bluebirds. Auk , 117 117: 1078-1080.

Dickinson, J. 2001. Extrapair copulations in western bluebirds (Sialia mexicana): female receptivity favors older males. Behavioral Ecology and Sociobiology , 50 50: 423-429.

Guinan, J., P. Gowaty, E. Eltzroth. 2000. Western bluebird: Sialia mexicana. Pp. 1-32 in The Birds of North America , Vol. 510. The Academy of Natural Sciences, Philadelphia, and The American Ornithologist's Union, Washington, D.C.

Kraaijeveld, K., J. Dickinson. 2001. Family-based winter territoriality in western bluebirds, Sialia mexicana: the structure and dynamics of winter groups. Animal Behaviour , 61 61: 109-117.

U.S. Department of Agriculture, 2002. "Index of Species Information: Wildlife Species, Sialia mexicana" (On-line). Fire Effects Information System. Accessed 03/01/04 at http://www.fs.fed.us/database/feis/wildlife/bird/sime/index.html .