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Sialia mexicanawestern bluebird

By Tiffany Musick

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. (Guinan, et al., 2000)

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. (Guinan, et al., 2000)

  • Range elevation
    2,900 (high) m
    ft

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. (Guinan, et al., 2000)

  • Sexual Dimorphism
  • sexes colored or patterned differently
  • male more colorful
  • Range mass
    24 to 31 g
    0.85 to 1.09 oz
  • Range length
    16.5 to 19 cm
    6.50 to 7.48 in
  • Average basal metabolic rate
    0.423 W
    AnAge

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). (Dickinson, 2001; Dickinson, et al., 2000)

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). (Dickinson, et al., 2000; Guinan, et al., 2000)

  • Breeding interval
    Females typically lay one to two clutches per year
  • Breeding season
    May to July
  • Range eggs per season
    3 to 8
  • Average eggs per season
    5
  • Average eggs per season
    4
    AnAge
  • Range time to hatching
    12 to 18 days
  • Average fledging age
    21 days
  • Average time to independence
    2 weeks
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

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). (Guinan, et al., 2000; Kraaijeveld and Dickinson, 2001)

  • Parental Investment
  • no parental involvement
  • 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). (Kraaijeveld and Dickinson, 2001; U.S. Department of Agriculture, 2002)

Home Range

We do not have information on home range for this species at this time. (Kraaijeveld and Dickinson, 2001)

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. (Guinan, et al., 2000)

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. (Guinan, et al., 2000)

  • 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. (Guinan, et al., 2000)

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. (U.S. Department of Agriculture, 2002)

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. (Dickinson, 2001; Guinan, et al., 2000)

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. (Guinan, et al., 2000)

Contributors

Alaine Camfield (editor), Animal Diversity Web.

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

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

cooperative breeder

helpers provide assistance in raising young that are not their own

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

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

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

Guinan, J., P. Gowaty, E. Eltzroth. 2000. Western bluebird: Sialia mexicana. Pp. 1-32 in A Poole, F Gill, eds. 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: 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.

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