Sialia currucoides

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.

• Biogeographic Regions
• nearctic nearctic
  • native native

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.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland

• Other Habitat Features
• urban urban
• suburban suburban
• agricultural agricultural

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.

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

• Sexual Dimorphism
• male more colorful

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.

• Mating System
• monogamous monogamous
• polygynandrous (promiscuous) polygynandrous (promiscuous)
• cooperative breeder cooperative breeder

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”.

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

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.

• Parental Investment
• male parental care male parental care
• female parental care female parental care
• pre-hatching/birth
  • provisioning
    • male
• pre-weaning/fledging
  • provisioning
    • male
• pre-independence
  • provisioning
    • male
    • female

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.

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.

• Key Behaviors
• arboreal arboreal
• diurnal diurnal
• crepuscular crepuscular
• motile motile
• migratory migratory
• solitary solitary

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.

• Communication Channels
• visual visual
• acoustic acoustic

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

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.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore
• herbivore herbivore
  • frugivore frugivore

• 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.

• Anti-predator Adaptations
• aposematic aposematic

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 ).

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.

Additional Links

Encyclopedia of Life

Contributors

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

References

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