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Sialia currucoidesmountain bluebird

By Tamana Amiri

Geographic Range

Mountain bluebirds (Siala currucoides) have a remarkably vast distribution, covering all of western North America, as far north as Alaska and as far south as central Mexico. They are also known to wander out of their normal range, especially when compared to their sister species, eastern bluebirds (Sialia sialis) and western bluebirds (Sialia mexicana). Their year-round distribution is mostly concentrated in parts of Oregon, northern to central California, Nevada, Utah, Colorado, New Mexico, and Arizona. During their breeding season they can be found over an estimated 5,567,500 km2 area in North America. This area spans across the northwestern United States, Canada (from Manitoba to British Columbia), and all the way north to Alaska. During the non-breeding season, mountain bluebirds can be found across parts of California, New Mexico, Arizona, Texas, Oklahoma, and south into central Mexico. (Daniel C., et al., 2004; J., 2015)

Habitat

Mountain bluebirds can be found in a variety of grassland-like and even savannah-like habitats, as long as trees and nesting sites are dispersed loosely. They are completely absent from densely covered forests and prefer vast meadows with scattered trees. They are especially common, however, in anthropogenically influenced areas such as clear-cut forests, areas burned by fire (as long as some trees still stand for nesting), and areas where there are widespread installments of artificial nest boxes (such as southwestern Manitoba). The early successional stages following a clearing or a fire are often dominated by colonizing grasses, creating ideal habitats for mountain bluebirds.

There seems to be no limit on the altitude at which mountain bluebirds nest - some have been found in trees at ~4,270 m in elevation (~14,000 ft). Nesting pairs often associate with trees such as conifers or aspens, with the greater nest abundance in aspens. In fact, the number of mountain bluebirds increased after human-driven removal of conifers from aspen groves in northeastern California and western Montana. (Brent R. and Ryan D., 2014; Fred W., 1948)

Physical Description

Mountain bluebirds are colloquially known as "the bluest of the bluebirds", with their feather coloration unadulterated by other shades of red or orange. They are relatively large, ranging from 15.5 to 18 cm in length, and 24 to 37 g in mass. They are sexually dimorphic, with adult males displaying brilliant cerulean and cobalt blue upper feathers. The throats and upper chests of males are also a rich and distinct cerulean blue, fading to a pale white toward the lower part of their bellies.

Female mountain bluebirds are not as striking in their coloration. They are typically grayish-brown to ashy-gray on their heads and backs. Their throats, chests, and upper bellies range in color from pale grayish-brown, ashy-gray, or faint orange. The rumps of female mountain bluebirds retain a patch of the characteristic cerulean color. Females are also similar in appearance to female eastern and western bluebirds.

Recently fledged mountain bluebirds are very similar in appearance to adult females, only darker and duller in color, with their breasts and upper flanks spotted white. (Jon, 1981; "The birds of North and Middle America: A descriptive catalogue of the higher groups, genera, species and subspecies of birds known to occur in North America", 1907)

  • Sexual Dimorphism
  • male more colorful
  • Range mass
    24 to 37 g
    0.85 to 1.30 oz
  • Range length
    15.5 to 18 cm
    6.10 to 7.09 in

Reproduction

Mountain Bluebirds are “socially monogamous”, meaning that individuals establish a social mate of the opposite sex to bond with for one or more breeding episodes, but sometimes copulate with other individuals. Copulation occurs daily throughout the egg-laying period, usually within 10 m of nesting sites. They are also cooperative breeders, often with juveniles from the first brood helping to feed the second brood. This cooperation increases the likelihood that shared genetic material will get passed on to future generations. Cooperative breeding is almost always associated with relatedness, although a few cases of cooperative breeding among unrelated individuals have been observed. (Daniel R., et al., 2006; E.A., 1931; Harry W., 1966; Harry W. and Michael P., 1996; Scott L., et al., 2013)

Mountain bluebirds pair and begin laying eggs as early as mid-April and spanning until early August. In many cases, females that successfully fledge one brood remain with their mates to fledge another brood in the same season. However, if females initiate their first clutch late in the breeding season, it is unlikely that they will initiate a second clutch.

Clutch size ranges from 4 to 6 eggs, usually having one egg laid per day until clutch completion. Upon laying their eggs, female mountain bluebirds typically incubate their eggs for 12 to 16 days during daylight hours. The incubation period begins upon the laying of the final egg to the start of hatching. Incubation is not constant over this time frame. Most females engage in about 2 to 9 irregularly spaced bouts of diurnal egg warming. Females incubate nocturnally as well, but the duration and onset varies greatly among different nests. Diurnal incubation is much more common. Only females incubate their eggs, but most males feed females near their nests during incubation. This is a distinct component of the reproductive behavior of mountain bluebirds known as “incubation feeding”. (Daniel R., et al., 2006; Scott L., et al., 2013)

  • Breeding interval
    Mountain bluebirds breed once yearly during the warmer months, late spring through august.
  • Range eggs per season
    4 to 6
  • Range time to hatching
    12 to 16 days
  • Average fledging age
    3 weeks

When it comes time for mountain bluebird eggs to hatch, they typically do so in the order which they were laid. Hatchlings are altricial and basically immobile, relying fully on parental investment in order to survive. Brooding is done exclusively by females for about a week, with variations in brooding period depending largely on ambient temperature and brood size. During this time males set out to find food. They typically pass this food to females so they can distribute it to the young. Nestlings typically fledge after about 3 weeks of parental care. During these three weeks, nestlings are almost completely dependent on parents for food and protection. As fledglings become more able to feed themselves, parents feed them progressively less to promote their independence. (Harry W., 1966; Harry W. and Michael P., 1996)

Lifespan/Longevity

Mountain bluebirds can live for up to 10 years, but adult survivorship is difficult to measure because mortality cannot be distinguished from permanent emigration. Survival rates of fledglings before independence has not been well-documented, and estimates of independent juveniles that survive to breeding age are also lacking. However, juvenile survivorship is very low among sister species such as eastern bluebirds (Sialia sialis) and western bluebirds (Sialia mexicana), with most mortality occurring within the first year of life. (J.A. and A.S., 2017)

  • Range lifespan
    Status: wild
    10 (high) years

Behavior

Mountain bluebirds are not considered to be highly social, with most intraspecific interactions being observed during non-breeding season. They are often seen in flocks before, during, and after migration, although not much is known about their migratory behavior. During breeding season, mountain bluebirds are highly territorial, with both sexes defending their shared territories after pairing.

Mountain bluebirds also engage in interspecific flocking with western bluebirds (Sialia mexicana) and western wood pewees (Contopus sordidulus) during the non-breeding season. (Alexander and R.B, 1914)

Home Range

Not much is reported regarding specific home ranges for mountain bluebirds, but they are known to defend territories around their nests during breeding season.

Communication and Perception

It has been disputed whether or not mountain bluebirds have distinct songs used for specific forms of communication. Detection of specific songs is difficult because most vocalizations occur during predawn hours. In addition, the sex of birds that were heard singing during these hours remains largely unknown, because it is often too dark to properly see their plumage. Some studies conducted in Montana confirm that it is primarily males that communicate with true songs. Additional observations made in Montana suggest that males in the region sing frequently throughout the day while eggs are incubating, and continue this behavior until young begin to hatch. The singing behavior of mountain bluebirds seems to vary regionally, with some populations singing more frequently than others. There is no confirmed recognition of females who produce strong songs.

The functions of mountain bluebird songs is relatively unstudied. There has been no systematic effort made to totally discern different songs, but specific sounds do appear to serve certain communicative roles. For example, the harsh “chup” calls they are known to make appear to serve as alarm calls, alerting mates to possible danger at nests. Some songs have also been identified as “advertising songs” or “territorial advertisements” because of their role in mate attraction and males counter-singing at distinct territorial boundaries, respectively. Males also use song as an "all-clear” signal to female mates, indicating that no predators are lurking near nest sites. A totally unexplored area of mountain bluebird songs is whether or not females use vocalizations to communicate with males from inside nests.

Aside from vocal communication, adults also snap their bills. This occurs when they are diving at humans (if they feel threatened), and if there are potential predators too close to the nest. (Harry W., 1966; Harry W., 1974; Weydemeyer, 1934)

Food Habits

Mountain bluebirds eat small seeds and fruits on occasion (often during the winter), but their primary source of food is small invertebrates. Mountain bluebirds are more carnivorous than most thrushes, feeding indiscriminately on insects such as caterpillars, grasshoppers, crickets, cicadas, spiders, and more. They strongly prefer to hunt on exposed, bare fields, where they can better spot ground-dwelling invertebrates (which are favored over arboreal and aerial insects).

The means by which mountain bluebirds capture their prey varies, but they generally rely on aboveground perches for spotting and dropping in on prey. Perch-foraging involves searching for prey on the ground from an elevated perch - most commonly tree branches or fence lines. Once a prey item has been spotted, mountain bluebirds drop to the ground to catch it. Although this is most common, ground-foraging and hover-foraging are also utilized to capture insects, although they are considerably more energetically expensive. In fact, among the three bluebird species, mountain bluebirds are the only species to hover-forage, which is possibly due to their lower body-weight to wing-area ratio. In addition to these intentional foraging habits, a more opportunistic strategy is fly-catching, where they catch flying prey on wing. Flying prey are usually spotted initially from perches. (Harry W., 1980)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • Plant Foods
  • seeds, grains, and nuts
  • fruit

Predation

Adults and free-flying juveniles are most commonly preyed upon by many large birds, including but not limited to:

Sharp-shinned hawks (Accipiter striatus), Cooper’s hawks (Accipiter cooperi), prairie falcons (Falco mexicanus), peregrine falcons (Falco peregrinus), and the great-horned owls (Bubo virginianus). Domestic cats (Felis domesticus) are also well-documented predators.

Predators of eggs and nestlings are quite different from those of more mature mountain bluebirds. The most well-known nest predators include raccoons (Procyon lotor) and tree-climbing snakes. However, rodents such as chipmunks and squirrels (family Sciuridae) and deer mice (Peromyscus leucopus) as well as mustelids like weasels (genus Mustela) have also been reported to feed on vulnerable nestlings. (Joseph S. and Richard M., 1937)

  • Known Predators
    • Sharp-shinned Hawk (<<Accipiter striatus>>), Cooper’s Hawk (<<Accipiter cooperii>>), Prairie Falcon (<<Falco mexicanus>>), Peregrine Falcon (<<Falco peregrinus>>), Great Horned Owl (<<Bubo virginianus>>)

Ecosystem Roles

Mountain bluebirds are mostly insectivorous, thereby curbing the population sizes of the various insects upon which they prey. This is especially relevant for bluebirds that live near agricultural sites because it leads to a reduced number of crop pests. Mountain bluebirds also host various parasites, including feather mites, nasal mites, ticks, and larval blow flies. Parasitic interactions are generally not detrimental, but nestlings affected by pests like larval blow flies had slower overall growth rates.

Mountain bluebirds are also an important food source for their predators, which include as the sharp-shinned hawks (Accipiter striatus), Cooper’s hawks (Accipiter cooperi), prairie falcons (Falco mexicanus), and peregrine falcons (Falco peregrinus). ("Bird Blow Flies (Protocalliphora) in North America (Diptera: Calliphoridae), With Notes on the Palearctic Species", 1989)

Economic Importance for Humans: Positive

The regulatory effects that mountain bluebirds have on their invertebrate prey is economically valuable in areas that are dedicated to grazing and agriculture. Insect populations that typically feed on crops are kept in check by predators such as mountain bluebirds, and therefore less investment needs to be made in pesticides.

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

There are no known adverse effects of mountain bluebirds on humans.

Conservation Status

Although population decline has been noted over the past 50 years, the degree of this decline is not alarming to conservationists. Their distribution is vast, with an estimated population size of approximately 6 million individuals. In 2016, the Partners in Flight and Science Team assessed their conservation vulnerability (based on current and probable future conditions), and deemed it unnecessary for them to be ranked as a species of conservation concern. Thus, they are listed as a species of least concern on the IUCN Red List and have no special status on the U.S. Migratory Bird Act, the U.S. Federal List, CITES, and the State of Michigan list.

Contributors

Tamana Amiri (author), California State University, San Marcos, Tracey Brown (editor), California State University, San Marcos, Galen Burrell (editor), Special Projects.

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

agricultural

living in landscapes dominated by human agriculture.

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

arboreal

Referring to an animal that lives in trees; tree-climbing.

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.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

cooperative breeder

helpers provide assistance in raising young that are not their own

crepuscular

active at dawn and dusk

diurnal
  1. active during the day, 2. lasting for one day.
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.

female parental care

parental care is carried out by females

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

insectivore

An animal that eats mainly insects or spiders.

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).

male parental care

parental care is carried out by males

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.

oviparous

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

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

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

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

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.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

visual

uses sight to communicate

References

National Museum of Natural History. Bird Blow Flies (Protocalliphora) in North America (Diptera: Calliphoridae), With Notes on the Palearctic Species. 26. Washington D.C.: Smithsonian Institution. 1989.

United States National Museum Bulletin. The birds of North and Middle America: A descriptive catalogue of the higher groups, genera, species and subspecies of birds known to occur in North America. 50. Washington D.C.: Smithsonian Institution. 1907.

Alexander, W., R. R.B. 1914. A list of the birds from the vicinity of Golden, Colorado. Auk, 31: 309-333.

Benedict C., P. 1979. Foraging ecology and habitat utilization in the genus Sialia. Pp. 165-190 in D J. G., C R. N., F R. R., K J. C., J J. A., eds. The Role of Insectivorous Birds in Forest Ecosystems. New York, NY, USA: Academic Press.

Brent R., C., B. Ryan D.. 2014. Avian response to mechanical aspen restoration in Sierra Nevada coniferous forest. Restoration Ecology, 22: 616–624.

Daniel C., B., L. Kirsten, L. Luciana, H. R. William. 2004. Landbird and waterbird notes from Isla Guadalupe, Mexico. Western Birds, 22: 616–624.

Daniel R., A., W. Matthew F., W. David W.. 2006.

Individual quality and food availability determine yolk and egg mass and egg composition in Tree Swallows Tachycineta bicolor
. Journal of Avian Biology, 37: 252-259.

E.A., M. 1931. Bird memories of the Rockies.. Boston, MA, USA: Houghton Mifflin.

Fred W., H. 1948. A nesting study of the Mountain Bluebird in Wyoming. Condor, 50: 216–219.

Harry W., P. 1966. Biology of the Mountain Bluebird in Montana. Condor, 68: 351-371.

Harry W., P. 1974. The Mountain Bluebird: Sex and the evolution of foraging behavior. Ph.D. dissertation: 1-405.

Harry W., P. 1980. The foraging behavior of Mountain Bluebirds-with emphasis on sexual differences. Ornithological Monographs, 28: 1-31.

Harry W., P., D. Christopher G.P.. 1980. Experiments on cuckoldry in the Mountain Bluebird. American Naturalist, 116: 689-704.

Harry W., P., L. Michael P.. 1996. Mountain Bluebird (Sialia currucoides). Philadelphia, PA: Academy of Natural Sciences.

J.A., L., L. A.S.. 2017. "Longevity records of North American birds" (On-line). Accessed March 17, 2020 at https://www.usgs.gov/centers/pwrc/science/bird-banding-laboratory?qt-science_center_objects=0#qt-science_center_objects.

J., A. 2015. An indicator of the impact of climate change on North American bird populations. Ph.D. dissertation, Durham University: 1-87.

Jon, D. 1981. The identification of female bluebirds. Birding, 13: 4–11.

Joseph S., D., B. Richard M.. 1937. Raptorial birds in the cliff areas of Lava Beds National Monument, California. Condor, 39: 97–102.

Scott L., J., N. Felicia M., K. Dovid Y., H. Rachel M., A. Allison. 2013. Variation in incubation effort during egg-laying in Mountain Bluebirds and its association with hatching asynchrony. Journal of Field Ornithology, 84: 244–252.

Scott L., J., D. Russell D.. 2020. "Birds of the World" (On-line). Accessed March 15, 2020 at https://birdsoftheworld.org/bow/species/moublu/cur/introduction.

Weydemeyer, W. 1934. The song of the Mountain Bluebird. Condor, 36: 164.

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