Mountain Bluebird Nest: A Few Things to Know

By Alfredo Begazo.

In this article, I delve into the nest, and nesting habits of the Mountain Bluebird (Sialia currucoides). I explore the intricacies of:

  • Nesting territory
  • Nest building
  • Nesting period
  • Preferred nesting sites
  • Reuse of nests

I also discuss intriguing nesting behaviors exhibited by male and female Mountain Bluebirds. Let’s see what I have put together.

Abandoned woodpecker cavities are among the preferred nesting sites for the Mountain Bluebird. Foto: Lou Ann Harris.

Mountain Bluebird nesting territory and tactics to attract mates

Male Mountain Bluebirds begin establishing nesting territories at slightly different times depending on whether they are resident or migratory. 

  • Resident bluebirds stakeout territories from approximately February 20 to mid March. 
  • Arriving migratory males begin establishing territories approximately from late March to Early April.

Field studies have revealed that both resident and migratory females swiftly select a territory with a resident male. Shortly after the pair forms a bond, courtship behavior continues, and activities aimed at initiating a breeding attempt begin.

How Mountain Bluebirds choose a nesting territory

Selecting a nesting territory takes some interaction between the male and female Mountain Bluebirds. 

Resident or migratory males identify suitable nesting cavities and establish a territory encompassing a few potential nesting cavities.

The male knows that the nesting sites within the territory are key elements to attract and keep a female. The female is the one making the ultimate decisions choosing the territory/male or move on to see what’s out there. 

As the breeding season progresses, the number of potential nesting sites and territories controlled by unpaired males decreases, leading to a selection of only few choices for many females.

From direct observation, females seem to make their choice based on various factors, including the characteristics of the nest site and the male’s own attributes. 

However, the weight a female bluebird puts in these factors leading to a mate selection remains an intriguing area of study.

Experience counts

Male and female Mountain Bluebirds use their past experience in their nest site choices. 

Males tend to return and establish territories where they were successful in quickly attracting a female producing offspring.

Females do the same. They are more likely to return to the same male and territory where they were successful the previous year.

This may end up in the same pair nesting in the same territory year after year. 

Nest Building of Mountain Bluebirds

Nest building is a critical phase in the Mountain Bluebird’s breeding cycle. It typically commences after pair formation, with nesting material being added to suitable cavities. When pairs have access to multiple nest cavities, they often distribute nesting materials across several potential nesting sites before ultimately settling on one.

The onset of nest building is closely tied to weather conditions. In warm and dry weather, nest construction can begin immediately, while cold and wet weather can cause delays of up to a month or more. 

How do Mountain Bluebirds build a nest  

The Mountain Bluebird, a vivid symbol of North America’s open spaces, crafts its nest with remarkable dedication. The female bluebird is largely in charge of building the nest. She can spend as little as two days to over a week to build a new nest.

Males do little to help with the construction of the nest. They only bring some nesting material. However, they spend their time protecting the territory and guarding the female from straying outside the territory. 

Typical Mountain Bluebird nest and eggs. Notice the difference in nesting material between the central cup and around the cup. Light blue is the typical egg color. In rare occasions a female can lay white eggs. Photo: Lou Ann Harris.

Characteristics of the nest

The female nestles a cup-shaped nest within or enclosure chosen as a nest site. The first layer or bottom layer is composed of coarse, dry grass stems. Subsequently, the female brings fine dry grass up to around the cup in the center of the nest. 

The nest cup, where eggs are laid is lined with a layer of fine  dead grasses, stems, plant rootlets and, if available, animal hair. Some females include pieces of plastic or paper, feathers or any other material that looks fine.

The size and even the shape of the nest are dictated by the space available in the nesting cavity. But the central nest cup maintains consistent dimensions.

Assuming plenty of space as in a nesting box Mountain Bluebird measurements area as the follows:

  • Depth: 13.5 cm
  • Width: 17.5 cm between the entrance and floor. 

The cup depth is approximately 7.5 – 8 cm. The cup maintains its depth as the season progresses. It tends to stay in shape relative to the outer areas of the nest. 

Length of the Mountain Bluebird Nesting season

Breeding phenology (season) of the Mountain Bluebird.

The Mountain Bluebird’s breeding phenology is determined by the rhythms of nature. As mentioned above, warm and dry weather prompt Mountain Bluebirds to quickly initiate breeding activities. Conversely cold and wet conditions can cause delays. 

I reviewed an 11-year study of breeding Mountain Bluebird in central British Columbia where ornithologists documented the length of the Mountain Bluebird’s breeding season. 

The length of the breeding season of the Mountain Bluebird is defined as the period between the laying of the first egg of the first clutch and the fledging of the last chick.

The study revealed that the length of the nesting season is rather variable, ranging from 96 to 122 days

Where do Mountain Bluebirds nest? 

The Mountain Bluebird nest in places that provide a sense of enclosure. The sites picked by nesting pairs varies from abandoned woodpecker cavities, crevices created by broken limbs, nesting boxes provided by bluebird enthusiasts and odd nesting locations 

Their favorite nesting place: Unused  Woodpecker Cavities

Mountain Bluebird’s favorite nesting site are unused woodpecker cavities. Among them, abandoned Northern Flicker (Colaptes auratus) Cavities are preferred over others. 

This preference is largely due to the fact that Flicker cavities have the right entrance size and chamber to accommodate an incubating female and a growing set of young bluebirds. Smaller woodpeckers excavate cavities of smaller entrances and smaller chambers.

An interesting study on the nesting preferences of the Mountain Bluebird found that in spite of a greater number of other woodpecker cavities available, Bluebirds preferred Flicker cavities as their nesting site.

The table below shows the number of cavities that researchers found in the study area available to Mountain Bluebirds and the percentage of cavities used by nesting bluebirds.

Cavity CreatorPercentage of Cavities natural cavities availablePercentage of Cavities used by Mountain Bluebirds
Red-naped Sapsucker52%12.0%
Northern Flicker33%58.0%
Hairy Woodpecker9%13.3%
Fungus/Insect-induced Decay6%15.0%

The study revealed that in spite of a greater amount of cavities excavated by other woodpeckers Mountain Bluebirds primarily opted for flicker cavities followed by fungal/insect-induced cavities. 

Interestingly, the Hairy Woodpecker sapsucker cavities used by Mountain Bluebirds had expanded entrances perhaps made by other woodpeckers or squirrels. 

This demonstrates that the size of the entrance and chamber is what determines Mountain Bluebird selection of nesting sites.

Cavities or enclosures created by fungus and insects are generally bigger than a flicker cavity.

Cavities of Northern Flickers have the ideal entrance size and chamber space for nesting Mountain Bluebirds. Photo: Mike.

Unusual Mountain Bluebird Nest Sites 

Mountain Bluebirds can be flexible when it comes to the lack of ideal nesting sites. Anything that resembles an enclosure has been used as a nest site. 

They have been observed nesting in: 

  • Broken branch nodes
  • Broken off tree stumps 
  • Cavities in dirt or sand banks 
  • Burrows created by Belted Kingfishers or Bank Swallows
  • Burrows created by chipmunks 
  • Crevices within boulders
  • Hollows of fallen tree trunks
  • Clusters of twigs in conifer trees
  • Nests of American Robin located under roofs
  • Structures created by Cliff Swallows and American Dippers
  • Wood and metal mailboxes 
  • Holes or crevices in buildings’ eaves and walls
  • Discarded roofing material
  • Farm machinery, and even grounded airplanes
  • Dryer vents
  • Large horizontal metal pipes
  • Wooden moose sculptures 
  • Railroad car couplers

Mountain Bluebird and nesting boxes

Mountain Bluebirds have a long standing relationship with humans. Dating back since the 18th century bluebird enthusiasts have been erecting “bird houses” or “nest boxes” for their bluebirds.

These nest boxes offer a range of advantages that can significantly enhance the nesting success of these stunning blue songbirds.

Do Mountain Bluebirds reuse the same nests year after year?

Mountain Bluebirds tend to reuse the same nest but the number of reuses of the same nest may be limited by the size of the cavity.

When a pair reuses a nesting cavity they build the new nest atop the soiled remnants of the previous nest. This practice limits the number of reuses as the cavity gets filled up with nesting material limiting the space available inside.

New nests can be built on top of nests that contain unhatched eggs and even the remains of chick that died in the nest. 

A study that used nesting boxes to measure how often Mountain Bluebirds reuse their nest found the following: 

  • 94% of females initiating second clutches within the same season remained in their territory. 
  • Of these, 63% chose the same nest box for their second attempt, while the rest opted for an adjacent box. 

Key Takeaways

  • Mountain Bluebird Nesting Strategies: Territorial males select potential nesting cavities and establish a territory around the cavities. Females chose a male and its territory to form a breeding pair.  
  • Nest Construction and Composition: Female Mountain Bluebirds invest considerable effort in nest construction, using primarily coarse grass stems to create a cup-shaped structure within the cavity. Feathers, plant materials, and even unexpected items like plastic may be incorporated.
  • Environmental Influences on Nesting: The timing of nest building is influenced by weather conditions, with warm and dry weather facilitating immediate construction. Cold and wet weather can delay nest building by weeks.
  • Nest Site Selection: Mountain Bluebirds show preferences over abandoned woodpecker cavities particularly those of the Northern Flicker. They are also very flexible regarding nesting sites to nest.
  • Nest Reuse: While some nest cavities are reused, repeated nesting can lead to shallower cavities due to accumulated nest material forcing bluebirds to look for another site to breed.

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