Eggs of the Mountain Bluebird: A Closer Look 

In this article, I explore some facets surrounding Mountain Bluebird eggs. I explore aspects such as:

  • Egg laying
  • Egg appearance
  • Clutch size
  • Hatching of the eggs
  • Second clutches

 I also discuss intriguing facts about the eggs of Mountain Bluebirds. Let’s see what I have prepared.

While most Mountain Bluebird eggs are bluish, in rare occasions females lay white eggs. Clutches are either all bluish or all white, there is no mixed bluish and white clutches.

Egg-laying, weather conditions, and migration 

The timing of egg-laying is influenced by a delicate interplay of factors such as weather conditions and migration schedules. These elements shape the timing decisions of these birds.

Spring storms and long periods of cold or heat influence the availability of food disrupting the onset of nesting and egg-laying. 

Field observations indicate that wet and cold conditions during the beginning of the Spring delay the breeding process. The delay can happen at any point whether pairs have already formed or not. 

Among migratory Mountain Bluebirds, those that arrive late to the wintering grounds,  begin breeding later than year-round residents and early arrivals. 

The initiation of egg-laying can unfold anywhere between:

  • Mid to late April, extending up to June. 
  • In the majority of locations, it typically begins in late April or early May. 

A study on the timing of egg-laying in central British Columbia found that:

  • The median date for the first egg of the season to be laid was found to be 27 April. 
  • First breeding attempts initiated clutch laying from 15 April to 6 May.

However, prevailing weather conditions and migratory patterns can affect the onset of egg-laying. In fact, the timing of egg-laying varies not only from location to location but also from year to year. 

The Latitude also has an effect on the timing of egg-laying by Mountain Bluebirds. As a general rule, lower latitudes (towards the south of the species range) initiate the egg-laying earlier.

Mountain Bluebird egg appearance

Most eggs laid by female Mountain Bluebirds are pale blue. However, on rare occasions females lay pure white eggs. There are no mixed-color clutches, that is, eggs in a clutch are either all blue or all white.

Egg measurements

Measurements obtained from multiple clutches show that the that:

  • Egg length ranges from 0.8 to 1.0 in 
  • Egg width ranges from 0.6 to 0.7 in 

Clutch size 

A female Mountain Bluebird regularly lays clutches of 4-5 eggs. Clutches of 6-8 eggs are uncommon to rare. 

Incubation of the eggs takes from 13-17 days with shorter incubation periods toward the south portion (lower latitudes) of the species range.

The following table contains a summary of the key breeding information about the Mountain Bluebird eggs.

Clutch Size4-8 eggs
Incubation Period13-17 days
Number of Broods1-2 broods

Hatching of the Eggs

The influence of incubation

I found an interesting study that explores the idea of warmth provided during the egg-laying period and the timing of egg-hatching.

Johnson and colleagues (2013) shine a light on the often overlooked role of incubation during the egg-laying process. He found that incubation effort is the silent force that plays a pivotal role in shaping hatching of the eggs.

Specifically, warmth provided to eggs laid early prior to the laying of the final egg has a significant impact on the hatching timeline. 

Surprisingly, the more warmth eggs received during the egg-laying period the greater the asynchrony in hatching timings. 

This may be due to the fact that during the egg-laying period females bluebirds incubate the egg in an erratic manner. Some eggs may get more incubation time than others resulting in asynchronous hatching.

The sequence at which Mountain Bluebird eggs hatch is influenced by the erratic incubating the female does prior to completing the clutch.

Timing of Egg Hatching and environmental conditions

Environmental conditions appear to affect the sequence of hatching of Mountain Bluebird eggs. 

Johnson and his team conducted a study in the heart of north-central Wyoming. They unraveled the spectacle of synchronized hatching. 

They studied 88 nests where hatching times of the first and last eggs were documented. They found that:

  • More than half, 51% of all eggs, hatched within a window of about 24 hours. 
  • The rest of the nest had a more extended hatching period spanning from 24 to 48 hours.

What is interesting here is that most of the 51% (45 nests) that hatched within a window of 24 hours were renesting pairs. These birds had lost their eggs due to a late snowstorm and initiated another nesting attempt later in the season. For these pairs this was the only brood for the season .

The remaining 43 clutches that took longer hatching periods were early nesters. 

At this point we know that second broods hatch within a shorter period of time than the clutch of early nesters. The reason for such a difference is unclear and a good reason for a study that addresses this question. 

Sequential Hatching Pattern

Observations in various habitats indicate a tendency for Mountain Bluebird  eggs to hatch in the order they were laid. This subtle yet consistent pattern suggests that hatching generally aligns with the egg-laying chronology.

Eggs that are laid first tend to hatch first.

Further investigations led by O’Brien and Dawson (2009) and J. A. Randall (2016) in central British Columbia shed more light on the sequential hatching pattern. In the majority of nests (65% of nests studied) where a single egg had hatched it was consistently the first or second egg laid. The pattern held when two eggs had hatched as well.

Unhatched eggs tend to be the last one laid

The same ornithologists found that the last-laid egg tends to remain unhatched.

Interestingly, of the 160 nests studied where all but one egg had hatched in most cases (91%) the last-laid egg remained unhatched.

Furthermore, among nests with two remaining unhatched eggs, 58% of cases featured the last-laid egg and the second-to-last-laid egg. 

Second clutches or renestings after net failure

Adverse weather conditions, such as cold and snowy weather, can lead to the complete failure of nests in a local area, potentially endangering the survival of the eggs.

Mountain Bluebirds are well adapted to nest failure. Parts of the regions where they nest are subject to sudden snaps of cold temperatures and snow storms that causes the nesting attempt to fail. The extent to which breeding success is achieved after a setback depends on the initiation of breeding and occurrence of inclement weather. 

Second Broods: Timing Matters

There is a clear link between the timing of a pair’s first clutch initiation and her chances of producing subsequent broods. This timing has a notable influence on the pair’s  breeding success.

Field studies on Mountain Bluebirds have unveiled a critical threshold date. 

  • Pairs initiating their first clutch after May 20th have a lower likelihood of attempting a second breeding cycle after successfully raising their first brood. 
  • When nest failure occurs after May 20th, pairs are less likely to attempt a second brood.

Trade-off of early egg-laying

As a general rule, early nesters are more subject to nest failure due to weather conditions than late nesters.

One may think that pairs are better off starting to nest later rather than earlier to avoid potential nest failure. The trade-off is that pairs that start earlier can have two broods in a season versus only one in pairs that start late.

Key Take aways:

Egg-laying, Weather Conditions, and Migration:

  • The timing of egg-laying is influenced by weather conditions and migration schedules.
  • Spring storms and extreme temperatures affect nesting and egg-laying.
  • Migratory bluebirds arriving late tend to breed later than year-round residents and early arrivals.
  • Initiation of Egg-laying:
  • Egg-laying can occur from mid to late April through June.
  • Latitude affects egg-laying, with lower latitudes initiating it earlier.
  • Mountain Bluebird Egg Appearance:
  • Most female bluebirds lay pale blue eggs, occasionally white.
  • Eggs in a clutch are either all blue or all white.
  • Clutch Size:
  • Clutches typically consist of 4-5 eggs.
  • Clutches of 6-8 eggs are rare.
  • Incubation period is 13-17 days, shorter in lower latitudes.
  • Hatching of the Eggs:
  • Incubation effort influences hatching timing.
  • More warmth during egg-laying leads to greater asynchrony in hatching.
  • Environmental conditions impact hatching sequence.
  • Sequential Hatching Pattern:
  • Eggs tend to hatch in the order they were laid.
  • First-laid eggs tend to hatch first.
  • Unhatched Eggs and Second Clutches:
  • Last-laid eggs are more likely to remain unhatched.
  • Unhatched eggs often correlate with the last-laid egg.
  • Trade-off of Early Egg-laying:
  • Timing of first clutch initiation affects chances of second broods
  • Early nesters face higher nest failure risk due to weather.
  • Early nesters can have two broods, while late nesters generally have one.

Literature Cited:

  • Johnson, L. S., F. M. Napolillo, D. Y. Kozlovsky, R. M. Hebert, and A. Allen (2013). Variation in incubation effort during egg-laying in Mountain Bluebirds and its association with hatching asynchrony. Journal of Field Ornithology 84:244–252.
  • Randall, J. A. (2016). Causes and consequences of blue-green eggshell colour variation in Mountain Bluebirds (Sialia currucoides). M.S. thesis, University of Northern British Columbia, Prince George, BC, Canada.
  • O’Brien, E. L., and R. D. Dawson (2009). Palatability of passerines to parasites: Within-brood variation in nestling responses to experimental parasite removal and carotenoid supplementation. Oikos 118:1743–1751.

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