Photo: John Brandauer /Flickr/CC by 2.0
One commonly asked question among folks that keep bluebird houses (nest boxes) is what to do with unhatched eggs when the rest have already hatched; or what to do with unattended eggs in a nest.
These questions also apply to nests outside birdhouses. Many bird enthusiasts, naturally, want to help when they perceive a problem.
This article goes through some basic information about bluebird’s eggs and nests that can help you make informed decisions.
Where do Eastern Bluebirds nest?
Bluebirds are obligate cavity nesters, which means that they only build nests inside a cavity, a chamber, or a structure that resembles a sheltered chamber.
Cavity-nester: A bird that exclusively or nearly exclusively nests in natural or artificial cavities.
A cavity is defined as a chamber with an entrance that shelters the nest, eggs, and brood.
Cavities can be natural, such as those excavated by woodpeckers, formed by broken off branches of trees, or artificial, such as birdhouses, also known as nesting boxes.
There are two types of cavity-nesting birds:
- Primary Cavity Nesters are those that are capable of making their cavity. A typical example is woodpeckers and Flickers. Some birds, such as Titmice and Chickadees can make their own cavities or expand existing holes in soft or rotten wood.
- Secondary Cavity Nesters are those that are unable to make their cavities. They are secondary users of either natural cavities or those made by others. Bluebirds, and most cacity nesting speceis, are secondary cavity nesters.
When nesting in natural cavities, Bluebirds use cavities excavated by woodpeckers 77% of the time.
When it comes to preference over artificial or natural cavities, Bluebirds have no particular preference. Field studies found that some pairs nested in a nest box and switched to a natural cavity, and vice versa, in a single nesting season.
On very rare occasions, Bluebirds have nested outside a cavity.
A literature review reveals that bluebirds, on rare occasions, have built a cup-shaped nest on a bank on the ground and in a fork formed by thick branches.
Allaire (1976) reported a nest on the ground on surface-mined lands in Kentucky. Sprunt (1946a) reports a bluebird nest on an oak limb in Clemson, South Carolina, and another in the longleaf pine (Pinus palustris) forests of Alabama.
Bluebird rarely nest outside a cavity.
Bird enthusiasts have reported bluebirds nesting in a gutter that has the appearance of an enclosure. However, nesting on the ground or trees has not been reported in recent years.
A substantial percentage of Eastern Bluebirds nowadays nest in birdhouses. Plenty of nest boxes are available to nesting bluebirds explaining why there are not many records of bluebirds nesting outside cavities.
Nowadays, there are more nesting sites available, but cavities of any type are always in demand.
Bluebirds compete for nesting sites with other secondary cavity nesters such as chickadees, titmice, nuthatches, wrens, great crested flycatchers, tree swallows, European starlings, and House Sparrows.
When do Bluebirds build their nests?
As a rule, Bluebirds that stay in their territories year-round, such as those in mid-eastern and southern warmer states, initiate nest building earlier than those that migrate to the northern and cold regions of the Bluebirds’ range.
Year-round residents begin nest building during February and March, the same period migratory Bluebirds are just arriving in the northern states.
In the states of Michigan and Minnesota, Bluebirds start building nests in March and late April.
The female alone builds the nest.
During the establishment of the territory and mating periods, the male performs a nest-building display. The brings some nesting material to the nest to lure the female into the cavity or birdhouse.
However, his contribution to the building of the nest ends there.
For birds that mate for life, Bluebirds do not share the nesting activities equally. The female alone builds the nest in approximately 5.5 days.
The nest is an open cup. The base is composed of longer, thicker, and generally more coarse nesting material, while the cup is lined with delicate fibers mixed with pieces of bird feathers, and mammal fur if available.
Some nesting territories have multiple cavities. Some pairs build a nest or the beginning of one in each cavity. However, the nesting pair use only one to lay the eggs and raise the young.
When does egg laying begin?
6 to 7 days after the nest is completed, the female Bluebird starts laying eggs. Approximately 13 to 14 days after nest building began.
The overall timing of egg-laying throughout the Bluebird’s range reflects the timing of nest building. Egg-laying begins during February and March in the warmer southern states and March and April in the northern and colder states.
During April, most breeding Bluebirds are at least incubating eggs.
How many eggs does a female Bluebird lay?
Bluebirds typically lay 4 to 5 eggs, though in some cases, they can have clutch sizes of up to 7 eggs. The female lays one egg every day.
On rare occasions, two females may lay eggs in the same nest. Both females incubate and feed the chicks.
Approximate length of the breeding season
Although variable across the Bluebirds range, the breeding season lasts 83 days on average or nearly three months.
When does egg incubation start?
Bluebirds typically start incubating the eggs on the day the female lays the last egg of the clutch. Less often, a female will start incubating the eggs a day before the last egg is laid.
The incubation time is slightly shorter in warmer regions and slightly longer in northern and colder states.
What is the bluebird egg color?
The color of the Bluebird egg is normally blue. But rarely some females lay white eggs or pink eggs only.
Egg color is not alternated within a single clutch. In other words, a female lays either all blue, all white, or all pink eggs in one clutch.
The incubation period is variable within the Bluebird’s ranging between 11-19 days with an average of 13.5 days. Bluebirds in the warmer southern states have slightly shorter incubation periods than those in colder northern regions.
Do male and female Bluebird incubate the eggs?
No, only the female incubates the eggs. The male does not participate in the incubation of the eggs.
The male protects the territory and brings food to the female while she is incubating the eggs.
The female takes breaks to feed and poop in the morning hours and throughout the day.
During these breaks, both the male and female forage near the nest and keep an eye open for predators approaching the nest.
In what instances do bluebirds abandon their eggs?
In most cases, bluebirds abandon their eggs for compelling reasons. Based on field observations, a mated Bluebird pair is more likely to abandon their eggs if either the male or the female has died. Lack of food and extreme weather events are also contributing factors.
Extreme weather events and lack of food are related. Generally, cold weather suppresses insect population growth and makes them less mobile and harder to detect by bluebirds. Therefore, bluebirds must spend more time looking for food, ignoring the eggs, or even temporarily move to areas where food can be found.
Repeated harassment by predators is another reason. There may be constant attempts by predators to get inside the next box. In spite of being unsuccessful, such harassment may prevent the female from incubating the eggs. Nesting pairs may not find it worthwhile to continue since eggs and nestlings may eventually be taken by predators and move to another nesting site.
Another less frequent reason is that the parents may realize that the eggs are not viable after incubating longer than usual. It is possible for them to abandon the eggs and move to another nest site, or to build a nest on top of the eggs and restart breeding.
Nest abandonment depends on when one of the pair is lost
Because male and female Bluebirds have specific roles during the breeding season, the timing of losing one member of the pair may result in different outcomes.
- If the female is lost during the nest building, egg-laying, and incubation periods, the breeding attempt fails because the male does not do any of these activities.
- If the male is lost during egg-laying through incubation, field observations indicate that the female will likely abandon the nesting attempt.
- If the female is lost when the brood is very young, the nesting attempt fails because the male does not brood the young, and they will die from hypothermia.
- If the male dies when the brood is very young, the female could raise the young.
- If either the male and female dies when the chicks are still in the nest but have feathers to thermo-regulate their temperature, the remaining parent can raise the young through fledging.
While any of these scenarios can result in nest abandonment, a likely outcome is that another bird replaces the missing member of the pair. The new replacement may help feed the young if these are old enough or about ready to fledge. Conversely, the new replacement is likely not to show interest in eggs or very young nestlings.
It appears my Bluebirds have abandoned the nest!
Sometimes long absences of the female from the nest occur when the temperature is warm or hot. The female stays outside and returns when she senses the eggs need to be incubated.
If the female has not been in or around the nest for two consecutive days, she may have died.
Another female is likely to replace the missing female. This replacement may take weeks or can occur within hours if females nearby are available.
The new female and male will proceed to start a new nesting attempt if it is early enough in the season to start one.
Photo: Jane Kirkland /Flickr/CC by 2.0
How long will the female sit on unhatched eggs?
The female Bluebird will sit on her eggs until the eggs hatch in 13 to 14 days.
If some of the eggs do not hatch within 72 hours or three days after all other chicks have hatched, it means something went wrong with those eggs and failed to hatch.
Studies on Bluebird nests indicate that about 83% of eggs regularly hatch. Typically, 17% of eggs never hatch.
If, for instance, all eggs fail to hatch in 13 to 14 days, the female will continue incubating the eggs for a few more days until she realizes the clutch has failed.
The pair then builds a new nest on top of the failed clutch and starts a new breeding attempt.
When only one or two eggs do not hatch but the rest do, the unhatched eggs are removed by the parents or are unintentionally crushed by the growing chicks. The female then proceeds to remove the shells.
How do you know if an unhatched egg is still alive?
A live egg releases CO2 and intakes oxygen through microscopic pores on the surface of the shell. If this exchange of gases is occurring, the egg should look smooth and shiny.
If the egg is dead, the surface is dull. If the egg has been dead for some time, it should feel weightless.
If the egg has recently died, a close sniff should reveal a foul smell.
Dead eggs of other birds ooze a light brown substance. If the Bluebird egg oozes a substance of any color, they are dead.
Use the candling method.
Candling is a standard method used to detect if chicken eggs are still alive.
Gently take the egg to a dark room and hold the egg before a light or flashlight. The light penetrates the eggshell allowing to see veins running through the egg or a red content indicating the egg is still alive.
Should I remove unhatched eggs from a Bluebird nest?
You should never remove unhatched eggs unless you are sure the nest has been abandoned or the eggs have failed to hatch (read above).
Why do some Bluebird eggs fail to hatch?
There are many possible reasons. Often we never know why an egg fails to hatch.
Some of the more probable reasons include too hot or too cold temperatures, pesticide exposure, genetic mal-formations, and bacterial infections.
Cold weather is a big problem for eggs and very young birds.
In general, bacteria may have found their way into the egg during egg formation inside the female. Bacteria continue to grow after the egg has been laid and eventually kill the egg.
Sometimes females lay eggs in another pair’s nest.
Bluebird studies where eggs were manipulated and genetically studied found that sometimes other females lay fertile eggs in another pair’s nest.
This is called conspecific brood parasitism, which occurs when a female uses or relies on another pair to raise their brood.
This may happen when a female with fertile eggs loses its nest to predation when laying eggs. She must deposit her egg somewhere rather than losing them.
Female Bluebirds guard their nests to prevent other females from laying eggs in their nests. Despite that, other females manage to sneak into other nests and lay eggs.
Bluebird eggs have been found in nests of other cavity-nesting birds such as Carolina Wrens, Chickadees, and even in nesting boxes of House Sparrows.
Different bird species may also lay eggs in a Bluebird nest.
The Brown-headed Cowbird is a known brood parasite. But parasitism by Cowbirds on Bluebird nests is rare when the cavity entrance is too small to allow a female cowbird easy access.
The same holds for nesting boxes. Nests with entrance an entrance hole of 5.4 cm (2.1 inches) in diameter and larger were parasitized by cowbirds more often than entrances with a smaller diameter.
How does the female Eastern Bluebird react to the presence of eggs from other bird species?
Most female Bluebirds that notice eggs from other birds will cover the entire clutch with nesting material, build a new nest, and start another nesting attempt.
What are the main Bluebird egg predators?
Bluebird predators that can damage the eggs include House Sparrows, House Wrens, and woodpeckers.
Predators that can kill young birds include fire ants.
Predators that can eat eggs, young birds, and adults include cats, raccoons, and snakes.
- Allaire, P. N. (1976). Nesting adaptations of bluebirds on surface-mined lands. Kentucky Warbler 52:70-72.
- Eastern Bluebird, Life History. All About Birds. The Cornell Lab of Ornithology.
- Gill, Frank (1995). Ornithology. New York: W.H. Freeman.
- Sialis Online. Bluebirds.
- Sprunt, A. (1946a). Unusual nesting of two birds in South Carolina. Auk 63:94-95.
- The Birds of the World Online. Eastern Bluebird (Sialia sialis). Cornell Lab of Ornithology, Ithaca, New York.