1. Female bluebirds regularly nest with a male but can accept an additional female in the nest
It is rare, but some breeding pairs have an additional female that helps them raise their young. It is also possible for two males to nest with a single female. A cooperative breeding system is also used by eastern bluebirds, whereby their older offspring help their parents raise their siblings from subsequent broods.
2. Female bluebirds regularly copulate with males other than their nesting partner
Scientists have discovered that mated eastern bluebird pairs consisting of one male and one female often have females that mate with more than one male, resulting in extra-pair paternity.
Therefore, males usually take care of offspring who are not genetically their own.
According to genetic analysis, extra-pair copulation frequently occurs, with sibling nestlings containing up from 7 to 20% genetic material from males other than the nesting male.
3. A female bluebird may lay eggs in nests other than her own
Laying eggs in other nests is also known as egg dumping or nest parasitism. In the bird world, nest parasitism is not rare.
Nest parasitism happens when a female bluebird lays eggs in another nest freeing herself from the responsibilities of incubating her eggs or raising the young. This means that some mated pairs can raise offspring that are not genetically related to them.
4. Females bluebirds ward off potential parasitic females
During nest construction and egg-laying periods, a female bluebird guards her nest closely.
These are the two periods when other female bluebirds are most likely to “dump” their eggs into other nests. Once the female finishes laying all eggs and sit to incubate them, the risk of parasitic eggs is minimal.
5. Female bluebirds defend their nests with their lives
Paired bluebirds often fight over nesting sites. Most disputes never reach a physical confrontation and end up displaying aggressive behavior until one pair gives up.
When a physical fight is inevitable, the male fights against the male while the female fights with the female.
In particular, female bluebirds are notorious for their fierce fighting over nesting sites. Because both females want access to the nesting site in dispute, they can fight to the point where they wound each other resulting in one female dead.
6. Females can establish nesting territories
When it comes to establishing a nesting territory, the most frequent behavior is that both males and females arrive together at the nesting territory.
Less frequently, males arrive first while the females delay their arrival to find males with an already established territory.
Females can also take the initiative to establish a territory by arriving first and establishing a territory with a suitable nesting site to wait then and recruit a male for the nesting season.
7. Female bluebirds lay eggs of different colors
A female bluebird typically lays blue-green eggs. Rarely she lays white or pink eggs. The female lays only one egg color, which means she never lays blue-green eggs and white eggs in the same clutch.
According to a detailed study of egg characteristics, 98% of females laid blue-green eggs, less than 2% laid white eggs, and less than 1% laid pink eggs (Siefferman et al., 2006).
8. Females do more work than males
A female bluebird expends more energy when caring for her young, including incubating, brooding, and feeding them.
The female lays the eggs and incubates them until they hatch. She then broods the young for the first week.
The time she spends brooding the young depends on the prevailing temperature. The female is more likely to spend less time brooding the young during periods of warm temperatures and plenty of solar radiation.
According to Buser (1980), a female bluebird spends an average of 47.3% of their time caring for the nestlings during week 1 after hatching, while the males spend 18.2%.
As the female incubates the eggs and broods the young, the male brings her food.
9. The female bluebird constructs the nest on her own
A female bluebird gathers every piece of nesting material from the ground to build a nest.
The male takes the first pieces of nesting material into the nest during the nest demonstration display, but that is pretty much his part in constructing the nest. The female then takes over the construction of the nest to completion.
10. Compared to the male, a female bluebird devotes roughly twice as much time to raising her young
In the first hour after hatching, both the male and female begin feeding the nestlings. Despite the similar food delivery rates by males and females, energy budget analyses indicate that females spend at least twice as much time and energy on nestlings as males do because females brood and feed their young. (Belser 1981, Buser 1980)
11. Female bluebirds are at a higher risk of predation
Bluebird nests are most often preyed upon at night and after the eggs hatch.
A female bluebird incubates her eggs alone, day and night. As soon as the eggs hatch, the young are fed by both parents simultaneously. Still, the female continues brooding the young for five to seven days—the feeding of the young increments the activity around the nest, which attracts the predators’ attention.
Since the female bluebird incubates the eggs and broods the young, she spends more time in the nest and is more likely to be found inside during a predator attack.
- Belser, C.G. (1981). Seasonal Ethograms of the Eastern Bluebird, Sialia sialis, near Clemson, South Carolina. MSc thesis, Clemson University, Clemson, South Carolina.
- Buser, C.S. (1980). A Time and Energy Budget Study of the Eastern Bluebird (Sialia sialis) during the Breeding Season. MSc thesis, Rutgers University, New Brunswick, New Jersey.
- Liu, M., L. Siefferman, Jr. Mays, H., J. E. Steffen and G. E. Hill. (2009). A field test of female mate preference for male plumage coloration in Eastern Bluebirds. Animal Behaviour 78 (4):879-885.
- Pinkowski, B. C. (1971a). An analysis of banding recovery data on eastern bluebirds in Michigan and three neighboring states. Jack Pine Warbler 49:33-50.